I agree with you that if it gets better, it should keep coming. Personally though, I don't believe it is getting better. Not saying that it's getting worse either. Brotherhood felt unnecessary and only had enough "new" features to barely qualify as something other than DLC. IMO the Ezio saga is getting rather boring, and I honestly believe that Ubisoft should just have stuck to the original idea of a trilogy only. From what I've seen Revelations is probably going to still be the exact same game as AC2 with one or two new shinies to keep it fresh, however I'd much rather see a bigger advance in storyline and more development in gameplay.
I'll still have to buy it though, the storyline completely suckers me in :/