All the great coaches from OhioHe turned down Texas...why would he go clean up a mess in LA? Only job I'd see him leaving for is OSU (he's from OH) or Alabama (top job in CFB) and I don't see either of those ever happening.
Like you said he's getting old, and if you know anything about Dantonio he's not a big city type of cat. Say he left MSU for a new gig, he'd make more sense at a program like Oklahoma than in Sothern California.
With that said he'd have USC in the playoffs by year 3...