This is absolutely, completely, and utterly flat out wrong. You're assuming that the pitchers you mentioned as part of the youth movement are not only ready to succeed in the show, but that they will be able to provide a full inning workload. They don't. That's why they have to push veterans off the team. That's what Bellhorn means. Getting rid of productive veterans for question marks is a fan's thought process, because fans don't have a direct grasp on a pitching prospect's development, and a GM can't afford the luxury of growing pains during a championship window.
The Red Sox don't have financial problems right now, so it makes zero sense to jettison any veteran pitcher (even Dempster) unless a prospect is literally setting the world on fire and forces the GM's hand. No current prospect has done that yet, though one of Ranaudo/Barnes may do that next year.