Everyone should take a few moments and think long and hard about what would happen if FFG produced a unified core rules set.
Step 1- Buy updated, unified core rules set. Don't expect as much content in each publication- in other words, expect to spend far more for the same quantity.
Step 2- Convert (through House Rules) every supplement that uses outdated rules, discarding sections that become redundant when the "splats" are released. (Many GMs will have already done this conversion work, resulting in no need for them to resort to Step 3, in which case buying an updated and unified rules set will be redundant, meaning many books will be printed without a genuine market demand, which in turn could drive FFG out of business.)
Step 3- Wait Tzeentch-knows how long for all supplements using outdated rules to be re-released with updated, unified rules. Expect glaring rules errors and contradictions, followed by yet more errata, and expect to spend more for the same (sketchy) quality.
If you only play Dark Heresy, then somewhere between steps 2 and 3 you'll spend upwards of $600 (again) on books that contain copy/paste text (complete with grammar and typographical errors) just to have updated rules mechanics. Black Crusade is a royal mess of rules loopholes, many of which found their way into Only War, so if you have amassed a complete FFG 40K RPG collection you'll spend well over $2000 just to have a complete set of updated and unified rules.
Quit griping about incompatability, ffs, and either do the conversion work yourself or spend some considerable time searching the forums and blogs for the conversion work that has already been done. Home-use printers, ink cartridges and paper are cheap compared to flogging your wallet for a revamp.
To FFG: Quit outsourcing your supplements, dedicate in-house writers and developers to these RPGs, and produce publications with consistent quality. I strongly suggest you go back and look at the production value of Black Industries' contributions to the 40K RPG, particularly Purge the Unclean, and rethink your corner-cutting ideology.