For better or for worst(worst for me), the main RE series is never going back to being solo only. They're going to cram in co-op til the series becomes unprofitable and /or dies.
The truth is that no matter how good the game actually is, co-op always seems to elevate games several points higher than if it were solo. People like playing games with their friends, even if it's mediocre or just bad. There's some pleasure to it.
That's why playing by yourself in RE5 or other games that were made with co-op in mind feel hollow/shallow in comparison to games specifically made to be played by yourself. You feel something missing.
Honestly, Dead Space 3 did it right/better, too bad that the story they told and the gameplay changes they made weren't very good. Someone else needs to come along and perfect that method. RE could do it. They probably won't though. Their games still sell butt loads.
I really don't think Dead Space 3 is a much better of example of balancing solo and co-op than recent RE's, honestly. Sure, you don't have to deal with a braindead ai partner, but it's still obvious that the level and encounter design cater heavily to co-op, and to top it all off, the game locks you out of a bunch of content when you're playing solo, which disgusted me, to be honest. I didn't hate the game, but I really don't see how it was any better of a single-player experience than RE5 and 6. It still seemed very co-op centric, to me.