From a multi-system customer viewpoint:
I think, that in an ideal world, the material would be distributed out packaged in as:
40k the RPG: Corebook (rules, gameworld, basically all of those that is the same or useable in more than one game line)
Gear book (all the gear, guns, implants and stuff you can buy & get that is useable in more then one gameline)
Dark Heresy -core (gameline specific careers, opponents, cults, daemons, heresies, subversive xenos, ..., the stuff specific & useable only in DH campaigns)
Dark Heresy Setting -books (Calixis sector, ...)
Dark Heresy Adventure -books (DH adventures)
Rogue Trader core (gameline specific careers, playing Xenos, pirates, hazards of the void, ship based adversaries, void kraken, ..., ships & all the stuff useable only to Rogue Trader -campaigns)
Rogue Trader Setting -books (Koronus expanse, ...)
Rogue Trader Adventure -books (RT adventures)
DeathWatch -core (space marine careers, the space marine-sized stuff, opponents fit for space marines, .... you get the point)
DeathWatch Setting (blah blah blah)
DeathWatch Adventures (blah)
Black Crusade -core (chaos careers, how to serve chaos, chaos only stuff, opponents of chaos, ...etc)
Black Crusade setting
Black Crusade Adventures
Only War -core (gameline specific careers, armies, enemies, ..., stuff really only useable in Only War -campaigns)
Unfortunately, this would be completely and utterly impossible simply due to the fact that it would require all of the material ever to be published to be written, tested, corrected, compatibilized, categorized and separated before publishing any of it.
Just the pure investment required for this would be absurdly staggering. Not to mention that customers generally aren't interested in buying games that require the purchase of several separate components to play, afaik. New costomers usually want to try with just the corebook, and will buy more, if they like what they get.
Okay, maybe it could be packaged as box sets, I recall buying those for RuneQuest in the 90's but would those sell today?
So, in practice, things are done the way they are for a reason, I suspect.
That said, what I'm really, really, hoping to get is official rules for playing ogryn, rules for all kinds of machines of war, artillery, aerial warfare, etc and good mass-combat rules.
But especially the ogryn rules. What kind of a Rogue Trader worth his salt wouldn't want his own low-grade but loyal spacemarine-substitutes for cheap? Also, playing Strongdumb Tuffington would be fun for a change since I usually play intellectual characters. Now if my group would just have someone to take over those duties ...
For Only War, I will reserve judgement until I have personally seen it. No point in jumping to conclusions, really.