The short answer is that the 40k setting has an enormous amount of room chronologically and geographically for more stories to be told without advancing the official time line. This is one of its greatest strengths.
As an addendum to that, the over arching story is not the life and death of the setting, it's support for the numerous table top games, novels, rpgs, card games, board games, short stories, and computer games which drive its existence. To assume that the story moving forward and flopping or not moving forward and stagnating will kill the setting is probably hysterical and ultimately false. Regardless, its making assumptions about market viability that are super speculative mostly meaningless.
If the questions becomes one of personal vision/story/interpretation/preference rather than franchise survival we can all happily pitch in without pretending our opinionating is something it's not.
As to my opinion, I too find Abnett and even Mitchell to be more compelling storytellers for the setting than most of the others, but I also am devoted to the idea that there is no canon, that all material could be propaganda, misinformation, confused records, misreporting, or blatant falsehoods. This too is a great strength of the setting and allows it to absorb even the apocalyptically bad material produced by Ward without suffering a fatal injury. After all, who is to say that reports of the Grey Knights bathing in the blood of battle sisters is not Chaos propaganda, misheard tales of the secretive Chamber militant of the secretive Ordo Malleus of the secretive Inquisition, or maybe just a transcription error in the records. Such flexibility makes me so much happier than swallowing all the Ward-**** as fact.
So does the time line need to move forward? Nope.
Can it? Sure.
Will it kill the setting? Nah.
Must it change it completely or be inadequate? Don't think that either.
The apocalyptic change here involves a great threat, but great threats liek the Horus Heresy et. al. did not destroy the setting, they added depth to it, they gave it history. Must this conflict's resolution end all conflict? I don't think so. The first Horus Heresy book has a line in which a certain famous Iron Fist tells a key Luna Wolf that "there is no ending the crusade, once it has conquered the Space Marines will still have to fight to preserve what they have gained, in the future, there is only war." No reason that can't be the case even with an Imperium victory.
A fundamental feature of 40k is that it allows cool armies to fight, as mentioned above. To maintain that you want to maintain all the armies existence. No problem. Here is a possible arrangement which allows for climax, without destroying all the factions.
Imperium - Have the Imperium survive, great epic (if not total) victory, climax achieved. Also, there, that faction exists (and by faction I mean lots of factions). And yeah, they still totally fight each other, so even if they are winning, a standoff (or dragged out victory with lots more battles for the setting) is possible.
Tyranids - Have the Tyranids get driven off, all but annihilated even. The Tyranids have a bounce-back mechanic, so wiping out all that exist now is totally irrelevant to them once again becoming a threat. Does this diminish the climax? Why should it? Does the end of WWI diminish the importance of WWII? We can always have more Tyranid fleets show up, or have fleets return after eating worlds somewhere safe and unnoticed. So, this faction exists too. Obviously if you want to end them as a major threat, that is in the cards too. Also, the Tyranids fight each other, so even if they have not been wiped out, a standoff is possible.
Necrons - Totally have the bounce back mechanic of "more wake up" super easy to brign back as a threat after defeated, minor or major, as desired. faction preserved. Also, they fight each other, so even if they have not been wiped out, a standoff is possible.
Tau - Who cares, they are a side-show pocket empire amongst pocket empires. if youw ant them to be major fine, have their communism spread like wildfire and unite humans/more aliens and vastly expand their power base. If you want them gone, super easy to make that happen. if you want them dominant as a ruling class, have them science up a massive tech advantage. if you don't care, leave them a sideshow. Regardless, faction preserved. Also, they can totally fight each other (you were mind/pheremone controlling us? *zap*), so even if they have not been wiped out and are somehow winning, a standoff is possible.
Eldar - These guys are super overrated in galactic power struggles, they seriously throw their whole population as militia at other factions to have an army, not exactly a winning plan when your birthrate is nil and your population close to nil. Still, if you want them to matter you deus ex machina distant craft worlds returning etc. if you want them to keep their "in decline" theme going, don't change anything. Faction preserved. Also, they fight each other, so even if they have not been wiped out but are somehow winning, a standoff is possible.
Dark Eldar - Yeah, scavengers on the periphery in some out-of-the-way stronghold? No reason to mess with this really. Faction preserved. Also, they fight each other, so even if they have not been wiped out and are somehow winning, a standoff is possible.
Chaos – Totally also has a bounce back mechanic, you can wipe them out, drive them into the eye, slaughter them all, heck you can do away with the eye itself and Chaos is totally plausible as a returning entity. Almost any amount of victory can be accompanied by a continuing threat of Chaos major or minor in its role. Also they fight each other, so even if they have not been wiped out and are somehow winning, a standoff is possible.
Orks – Also with the bounce back mechanic, also with the fighting each other. Also with their role being major or minor.
As far as victory for the Imperium changing it fundamentally I am not sure it would need to, driving off its many enemies might be a unifying moment, but the spoils of victory could be squabbled over. Even if the Imperium is re-energized it's still gargantuan, no amount of vanquished foes really ends their enormous logistical problems and constant brush wars. One can Deus Ex Machina an end to these things if one wants to, but why?
As to how the victory might come about I can conceive of amny plausible scenarios including the aforementioned propensity of every other faction to shoot each other in the back. The Imperium has enormous resources and large quantities of hero fodder, grinding that entire machine into motion can win by sheer numbers, honestly the only thing stopping even the Tyranids from being outnumbered by the force of man is organization. I am highly amused by the image of Tyranids being overwhelmed by waves of Guard (also can I just say that every calorie spent moving, thinking, fighting, shooting, etc. it bio matter expended, the Tyranids are not a perfect closed system, they need fuel and their own losses are still losses, even if they can recycle parts, what makes them work is getting to eat defenseless masses, a planet full of algae is a Tyranid gold mine, a defended planet is really not, espeically one with no eco-system left – 40 billion people on a hive world is still less bio-mass than the insects on our earth). Furthermore, the Necrons have a time bomb in the form of the current fluff's explanation of the C'tan as betrayed and basically enslaved. Let them get desperate and release too many shards on the battlefield and who knows if a freed and vengeful C'tan doesn't burn out half the Necron armies before succumbing? The Emperor returning obviously has all sorts of potential for Deus Ex Machina, as does his being reborn. The Imperium is honestly just one major STC discovery away from getting the upper hand (imagine a fully-functioning STC factory for TDA, or titans). But honestly, just having the Imperium pull it out in a big effort with lots of players and lots of close battles and lots of internecine conflict and eventually prevail is great fodder for extending the time line, letting us have our climax, and still letting the Imperium win, no need for the Deus Ex Machina at all really.
Really the only feature that messes with the setting so much that it will fundamentally change to storyline would be the Golden Throne/Emperor Reborn/ Emperor Revivified/Emperor Dead/Astronomicon waning plot line, and that need not truly transform everything about the setting. If the Emperor gets off the Throne and starts running things again there is no reason to assume that he will be of the same mind he was 10000-12000 years before. One can easily replace Emperor entombed in Golden Throne with Emperor sits on Golden Throne and carry on. The psyker sacrifices could even keep fueling the Astronomicon, now operated remotely by a walking, talking Emperor who is totally busy trying to conform institutions that have become engrained for millenia to his will, or maybe he is damaged or paranoid after the betrayal and his vision is rather less beatific, or maybe whether he is whole in mind will be the parallel in M.42 to whether he is alive or dying in the previous ten millenia.
TLDR: The time line need not move forward, but the themes and factions of 40k are totally preservable even if the time line moves forward, a climax is had, and details change.