Oh boy...
Only because Xenoblade has proven to be big, people wanting a rep in, and because it was one of the big new Nintendo IPs on the Wii.
So your criteria for supporting the original protagonist is... they have to be big (i.e. successful)... people wanting them in (i.e. popularity)... and because it's a new IP (which matters **** all to Sakurai, he doesn't give new IPs priority. If anything, most aren't proven enough to him... or i.e. recency).
Well let's see here DSN... you support Matthew over teenage Isaac and Isa over Saki, do you not? Yet teenage Isaac is more popular than Matthew (more people wanting him), both Isaac's games were "bigger" than Matthew's by at least three times. Saki is also more popular than Isa, and his game's release on the VC was the whole reason the second, which bombed hard btw, was even made. Even on the VC, it was more successful. So both original protagonists of these two series are more popular and more successful, and in Isaac's case, were the protagonist more than once (even if shared). They meet two of your three criteria, yet you support the other candidate. Hmm.
As for recency, yes, that is where Matthew and Isa are the choice, but one, Marth should prove recency doesn't take top priority, and two, wouldn't that mean you should support the protagonist of X, if it is directly related to Xenoblade, over Shulk? I mean, recency is the reason you support the other two, you've convinced yourself it's the determining factor to Sakurai (which... it isn't), you can't just abandon your logic when it doesn't suit your bias DSN.
If you want to stick to your old logic, there's no reason why the Shulk would get in over the X protag, unless teenage Isaac and Saki also get in over their counterparts. You contradicted yourself majorly, and put a huge whole in your logic. The only defense now really is to admit your bias, which was pretty obvious from the beginning anyway...
On top of that, it might be too late. And even if not, I wouldn't support it (at least not entirely).
Sakurai was adding characters last year. He goes to, or at least consults, the development teams when adding characters. If he were to add Shulk, he would've had to have come into contact with Monolith. Last year X would've existed in some stage, Sakurai has been aware of it since last year, if he did contact Monolith at all for Smash. Not that I think the new protagonist would be added over Shulk either (for very different reasons), but it's not too late.
Sorry about that, it happens sometimes
...it shouldn't. Makes your opinions looks even less credible than before, which is saying something.