Well, technically he's actually asking 2 different questions here -
Can he be a legit main eventer on Smackdown? Hell yes!
Can he be the "face" of the brand? No he cant.
He can be a transitional face champion, a legitimate threat to ME players, and a regular in ME storylines. But he cant "carry" the brand like Cena does on Raw, and Mysterio and Taker have on Smackdown in the past. He's just not that over/marketable.
Even his peeps should understand that he won't be the face of anything. As talented as he is, he just doesn't have that star power that guys like Cena/Orton/Mysterio/Edge possess. I'm happy with him just winning the title and having a decent reign for a couple of months.
He's as talented as Edge...Edge was a bit bigger, but Christian is awesome on the mic (better than Edge? Maybe, really good anyways)...excellent in-ring....he's over right now, getting a main event shot too...
So yeah, definite potential to make it. We'll see if it happens tho
A safe business move would be to have an Established Main eventer head to Smackdown But still have Christian accomplish the main event status.
It's his time to shine. And yes he can carry the Smackdown brand.
He's proved himself time and time again and has earned his way.