I have to disagree with you there. The players that have stuck 3-4 years with Pitino have improved a lot. That has not been his problem. His problem has been that he has had so many misses on the recruiting trail who were not able to cut it or had behavioral issues especially early in his stay here:
Problems: McNeil, Lofton, Dorsey, Lynch
Health or injuries: Curry, Johnson, Fitzgerald
Bad fit or lack of ability at this level: Martin, Gilbert, Konate, Gaston, Harris, Washington, Buggs
Good development: King, Walker, Austin Hollins, Murphy, Coffey, Mason, Oturu (big jump already).
Not much development: McBrayer and Hurt. McBrayer was developing but perhaps his injuries ended up derailing him. Hurt knows where to be on the court and has gotten better but not at the level one would hope.
Finding the right roster fits, building a full balanced roster and keeping guys 4 years has been the biggest issue with Pitino. I think that is getting better although the Harris, Washington class was a disaster (did get two real good transfers in Carr and Willis).