I agree with everything you've said there except at the end of the Fergie era it was really Vidic and Evans that were first choice.
I think the way Moyes handled the centre backs was his biggest failing at United. Vidic and Rio left in the name of "transition" yet Smalling and Jones played far too many games in other positions and Evans managed to be less established at the end of his reign than he was at the start. No matter who the manager was going to be this season, they would start the season with zero established centre backs.
It's a big challenge but also a big opportunity for all of them. They deserve the chance to succeed or fail. I hope the fans are patient with them. Hopefully at least Smalling and Jones can step up.
You make an excellent point about centre backs last season.
Moyes knew full well that Rio and Vida were leaving, he should have been making far more of an effort to bring Jones, Smalling and Evand though than he was.