Touche on those two, but even as fast as Sonic is he isn't always the smartest guy around. I've played nearly every Sonic game and love the series, even the bad ones, so don't think that I'm just hating on Sonic here.
On the other hand, doesn't Subspace have A LOT of plot stupidity as well? Including Sonic's introduction, which could have been so much better?
Well here's how I look at it - yes, it was disappointing that Sonic only showed up at the very end of SSE (even if what he did was awesome in hindsight - he saved Nintendo!).
However, keep in mind that Sakurai himself created Tabuu. So if Sakurai didn't think Sonic was capable of stopping him, why would he go out of his way to make that his role in SSE? The "he was rushed in" excuse doesn't work, since Sakurai could have just as easily threw him in elsewhere, or even made him a secret character within SSE like Wolf, Jigglypuff, and Toon Link.
It should also be noted that a lot of people seem to think that the entire SSE cast is needed to defeat Tabuu. But that's actually false. When the cast gets turned into trophies, you can skip them while going through the levels they are located. Only 6 characters are mandatory to beating the game, and those characters are Kirby, King Dedede, Luigi, Ness, Bowser, and Sonic, as they are the only ones who get revived as part of the plot (or in Sonic's case, he is never turned into a trophy at all). So we can assume that any one of these characters could defeat Tabuu alone, since it's possible to do so in-game, and these characters are greatly inhibited in-game anyway compared to their canon abilities (moving on a 2D plane, Sonic being much slower than normal, Luigi being restricted to moves that resemble Mario's, etc).
So no, I really don't think Tabuu is that big a threat. His whole plan in SSE was not something he achieved in a second, it required careful planning. Tabuu is powerful enough as a fighter, but I don't think he's that huge a threat if we give these characters their canon arsenal. He's more of a tactician, IMO.