Like, people use the change from 3e to 4e as an example of how bad and horrible huge overhauls are. But they seemingly forget that 3e itself was a massive departure from 2e, rendering most older books totally useless (as far as the actual mechanics go, anyway). You know, exactly what 4e did.
I think is a good example for a different reason...
Firstly, there was a big time gap between the major releases of AD&D and D&D 3e (and honestly, a lot of people had abandoned AD&D for other games... a big push early in the D&D 3e was the attempt to win people back to the game... the d20 Wheel of Time and d20 Call of Cthulhu games were just such efforts). D&D 3e was followed by the fairly rapid and minor revision between 3.0 and 3.5, different but not terribly so. D&D 4e seemed for come very hot on the heels of 3.5 and was a massive change in system to a game that still a fairly solid base audience. The fairly short perceived time between editions, combined with a fundamental change in direction and, seemingly, target audience, after a more incremental change, made the "edition shock" more extreme.
This is a very good analogy for the development of the 40K-RP line... each game within the 40K line can reasonable be seen as a different "edition" of the same rules set. We started with Dark Heresy and have seen minor, incremental changes every few years. That makes a massive change much harder for the established audience to accept.