Quite a few of the NXT have come up to the main roster and done quite well and had no trouble breaking through. 3 of them are in the main event scene right now. Rollins, Reigns and Ambrose were all products of the NXT system, and so were Dallas, Rose and Paige. I'm sure there is someone I've forgot, but anyway.
As a matter of fact, Ambrose won the US title, Reigns and Rolllins held the tag titles, and Paige is in the best Diva's feud in a while with AJ. She also won the Diva's title. Sin Cara has gone down to NXT and won the tag titles over a team that destroyed the locker room.
So I don't think that there is a problem making the main roster. I mean it's not getting any younger is it? They need to start bringing these guys up and getting them involved, and they need to do it soon.
EDIT: Oh I forgot Rusev, not surprised about that though.