I really, really don't understand this argument. It seems to be born of seeing changes to the system and not actually making any sort of logical evaluation of how that affects backwards compatibility.
I've noticed what most people who argue this case seem to want is essentially Dark Heresy updated to the Only War rules standard. That's fair enough, I suppose. Now, let's look at how backwards compatible Only War is with old Dark Heresy content.
Career paths, like the Sororitas stuff? Totally out, Only War uses a new advancement system.
Alternate career ranks? Same deal.
New psychic powers? Totally unusable without significant modification, thanks to the changed psychic power mechanics.
Home Worlds? Potentially useful, but the changes make things iffy at best, really. Still needs significant conversion work.
What this basically leaves us with is gear. Gear is the only area where content from DH splats is really compatible with Only War, or really anything else in the line save for Rogue Trader. Most gear is easily compatible between the games, it just means that some skills might need to be changed (because of the condensed skill list Only War has), and some weapon values might need to be tweaked to fit in line with the new content.
The thing is, the exact same thing is true of trying to convert DH1e content to DH2e. All of the things that wouldn't fit into Only War have no hope of fitting into DH2. Gear, however, fits fine, with some slight modifications. Skills have been changed, but that's simply a matter of adjusting bonuses given to suit whatever new skill the old skill has been folded into. Easy work, and you'd have to do it for Only War, too.
Weapons are really the only point I can see people legitimately arguing on, but converting those is painfully easy too. As of the first errata, weapon stat lines are incredibly similar to how they were in the previous system. Damage values are very similar, so are pen, rarity is just converted into a flat number but largely the same, etc. Literally all you have to do is assign AP costs (a ten second job of comparing against current tables) and adjust weapon qualities (much like you would have to do for converting to any of the newer 40kRPG systems, considering weapon qualities change every system).
Sorry for the huge rant, but seriously, help me understand. What are the so called backwards compatibility issues here? Is ten seconds of work to convert that Carnodon Hand Cannon really that much work for you? If so, maybe you shouldn't be GMing in the first place.