I wonder, if Nintendo made the next real Zelda multiplayer co-op (like 4 swords) would you guys be happy? It just seems strange to me that people are willing to accept multiplayer co-op in a series that has never had it besides a spin-off (and fun) online game. I'd think you could innovate the series without taking the multiplayer route like RE4 was able to do. Personally I am saddened by the single player experience starting to become a dieing breed. It makes me a sad panda. 
The only difference is that in Zelda, you only play as Link. Resident Evil lends itself more to multiplayer than Zelda.
I get what you're saying though and honestly, the only reason this happened is because Capcom top brass raised the pimp hand to Shinji Mikami and basically forced him to leave the company for who knows why considering what an asset he'd be to any development team. Not saying RE5 won't be a good game, but like you, I would have preferred the game staying single player only. The series isn't survival horror anymore. RE4 gave you tons of ammo and now RE5 gives you tons of ammo and a partner who, useless or not, makes the game less scary. They might as well add in some upbeat J-Pop while their at it.