While it's great to see more effort in the single-player, it's still a finite experience - it will end, and people won't play it forever, as opposed to the multi-player the game will contain. I will personally play through the single player quite a bit, but I'll stop at some point (after everything is unlocked), just as I did in Melee. Playing with friends on a multiplayer mode will ultimately consume the vast majority of time on this game.
With that said, I really hope that they put even more effort into multi-player. As good as single-player is, Smash is - at it's core - about multi-player, pure and simple. I don't mind if the single-player turns out to be a disappointment, as long as online is working fine.
Looking at how good the single-player is for this multiplayer game, I'm betting the multiplayer is going to blow us away.