You're making an assumption that the change introduced is supposed to keep DH power level comparable with that of Only War. It would have been a failure of design if it had been the goal all along, but I see no proof, explicit or implicit, that it's the case.
You're also ignoring an emergent trend in the sourcebooks since Blood of Martyrs. While you see the "naked power creep", you ignore what for a lack of better word I'll call "coolness creep" following. In the early days of the line, it was all about being a random, meaningless dude cast into the vast machine of the Inquisition. Even if you got an alternate rank or a background package, you were still pretty much the same random, meaningless dude, only now with some preset backstory and perhaps a small perk to show for it. Ever since BoM, the spotlight is on dudes who belong to important, elite organizations or were given elite Inquisition training, and have the skills and equipment to show for it. Now it's all about being a part of the Inquisition, a truly exceptional dude pulling his weight in this giant and tremendously powerful machine.
Practically speaking, we have two distinct "tiers" of playing DH now (Ascension notwithstanding): you can still play as the random assembly of random Acolytes, or you can play as the lean, mean task force carefully assembled by the Inquisition. The material for both playstyles is all over the place, but generally you have pre-BoM books offering most to the former playstyle and post-BoM books offering most to the latter.
I'll give to you that it does create a sort of conundrum due to not being spelled out clearly, and sticking to the old stuff while someone else in the team is rolling a Venator or a BoM-Battle Sister may not be the most fun play experience, but in a team consisting of, say, a Verispex Adept, a Venator, a Battle Sister and a Lathesmaster, I really don't think anyone would be truly overpowered.
Running this kind of game might not be your cup of tea, but it's a legitimate, fun playstyle, and again, the game doesn't actually break because of it.