You said about Roman that they intentionally Book every one down, to make him the only viable option. Why not Braun though? Before he was crushed by lesnar, Braun lost one match all year (to Roman), and got his wins back twice on ppv! Did they even try doing any Braun merchandise?
How about AJ Styles? His high profiles jobs this year were in a 5-star match to Cena and that's really it. On SDL he's protected, losing only via flukes and in multi-man matches. He sells merch, and has a huge fan base and can be marketed as an anti-Reigns alternative.
My other question is this; if the worst case for Vince happens, and Roman "Big Cass's" his knee… who gets the spot? Who gets the superhero build towards Brock?
Then you panic and bring Cena back to the spot, or use Orton. And then start the build to the next coronation of Roman again.