i don't see how online coop is a problem. if anything it made the game better, more fun and gave it more replay value (playing with different people, etc).
if it's because it removes the horror aspect, well, re wasn't ever really scary after 3. not cvx, not 4, both of which were single player. they were more like butt-clenchers, meaning you'd tighten muscles in suspenseful situations lol.
maybe i'm just a bit older now, so things like movies and video games are naturally less scary.
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.