I would disagree.  I've always enjoyed the RE series since the first but after RE5 my world of it has changed (gameplay-wise, however setting-wise I would prefer back in the city with real zombies).  Coop campaign is just an optional feature since you can still play RE5 & 6 by yourself I don't see the harm in having the option.

Personally i've always thought that it'd make more sense having coop since back then for RE.  Ie. When Claire and Leon met up why would they split up?  In no horror scenario would anybody ever want to split up.

RE5 introduced my gf into playing the series (she knew about the older ones but had no interest in them) and she loved it and is excited to play RE Revalations 2 with me.  Clearly we have different definitions of what is suited and can ruin the game.  Personally from my experience and those around me coop campaign made the RE series a whole lot better.