Celtics have their own "college all star" team right now. Our 'young developing guys" are doing just that and the squad has a solid 23 wins to show for it.
There's nothing solid about 23 wins (on track for 26-27).
Sullinger and Olynyk look like nice enough players, and Bradley, if he can stay healthy, will probably be a solid role player in the league for a while yet.
It's not entirely clear that any of the younger players on the Celtics are definitely starter material. Sullinger is probably the best bet for that, but I could see him ending up as more of a 3rd big in the rotation. Pressey, Babb, Johnson, Faverani . . . any of these guys could drop out of the league before next season and not make it back.
Rondo is the only established above-average talent on the Celtics right now, and barring some moves to get better ASAP, he may very well leave in free agency next summer.
Bottom line: the Celtics need to add talent any way possible.