I don't think RE5 sold well just because RE4 was awesome, look at RE3 it sold less than RE2 and it was obviously inferior and half the lenght while RE5 sold better than RE4 (even with a huge PS2 install base).
Capcom just tried to keep up with the times deliver a great game and make a nice profit, I like RE5, I think is great as I play it (like a co-op buddy 80s Zombie Movie, me and my brother cracking jokes at the ridiculous story) but I don't think they can do that again with RE6 it will fell "franchise fatigue".
PS: Sorry for my english not my first language







