In and of itself online co-op isn't really a problem. But I suspect we lost content due to online co-op. I also suspect the graphics took a hit in order to support online co-op. As far as replay value goes, IMO RE4 has the most replay value of any RE game so far with its very addictive Mercs. Even after unlocking the HC, I kept wanting to play to improve my high scores.
4 had the best campaign out of all the modern Resident Evil games, while 5 had the best Mercenaries mode hands down..also the graphics are amazing in 5, beautiful game. =]
I hope the future games are more like 4 'n' Revelations, epic single player campaigns, and co-op only included in Mercenaries, or raid modes..that is my wish. XD