I don't get what you are saying. If we have a proven third down back, he won't take carries away? If we have a proven goal line back, he won't take carries away?
Say Cross gets the same amount of carries as last year. We've already heard Ameer's words himself that Terrell is probably the most talented back in the room, so he'll likely get more carries than he did last year. Adam Taylor redshirted but has been drawing a good amount of praise so there's one back that can possibly contribute in meaningful snaps that we didn't have last year.
None of them are proven every down backs, as you say, but Ameer will likely get at least 2-4 fewer carries per game minimum, which could be over 50 carries lost on the year.