Well, he has become a starter because he has won it against his competition by putting in performances. He managed to get rid of his injuries or are playing through them more often, either way he stays fit to play for large periods of time which has given him more opportunities to prove himself, and in return his consistency has improved to that of a top CB. If you have talent, getting that consistency and reliability is what makes you. For a player like Smalling who lacks a few of the qualities that a modern CB is expected to have, to have that consistency in his defensive performances is what separate him from midtable run of the mill CB's. Does that make him top of the line world class? Not necessarily, but it makes him capable. It makes him valuable to big clubs, even if he is a defenders defender more than a Ramos type.
Hypothetical situation; If we get two worldclass CB's in and Smalling is second fiddle I don't think anyone would complain. I think however if Smalling won his place against one of them, it wouldn't be about how Smalling improved and won his place, but about how that other player wasn't really world class to begin with and we're back to "mid-table" level with Smalling, no matter how he plays.