As far as I'm concerned, complaining about single-player modes in Smash Brothers is like complaining about single-player modes in Mario Party: the game is meant for multiplayer and single player is just an afterthought. If anything, the SSE is a great example of why they shouldn't even try.
The problem is that Nintendo doesn't have a great track record of providing ways to play that game multiplayer. They ignored online play in Melee, and Brawl's online was
notoriously abysmal (not to mention, it relied on Nintendo's crappy Friend Codes system). And honestly, the same ol' multiplayer gets really tiresome after a while, and sometimes just don't want to
deal with other people and their issues.
I liked what Subspace Emissary and Melee's Adventure mode tried to do because it provided a different experience with the same rules. And after Mortal Kombat provided such an awesome story mode and an
extensive (and constantly changing) Challenge Tower, I'm not accepting excuses that MP-focused fighting games can't have good single-player content. Hell, Subspace Emissary
might have worked if Sakurai had focused on taking advantage of Nintendo's vast catalog of enemies and game worlds (rather than just making up generic ones to replace them).
I think when you put out a game with single and multiplayer content, it's your responsibility to ensure that
both are well-thought-out and fun. So far, Nintendo's half-assed every Smash's SP content in favor of the MP content. Eventually, I lose interest in the Multiplayer, and since the SP is crap I now don't have a reason to play the game ever again. Nintendo shouldn't give up on making a great, well-rounded Smash Bros. game just because they've failed before. Cutting features isn't "solving" a problem. It's running from it.