I am not sure if there is a problem here or not. We developed Tatum, Brown, Smart, RWill, and GWill just fine. And you could throw Pritchard in on the "good" side of the ledger also. Langford and Nesmith not so much. Tacko Fall and some others don't even matter.
Is there really a problem here and if there is, is it a problem with development of the talent or the talent itself? I don't think it is correct to say that a young player should be given playing time in order to develop. It is fair and correct that they have to earn the playing time.
If there were cases of young players failing in Boston but then going on to other teams and being properly developed and successful, that would add some credence to the argument. But I can't think of a case of that happening. Do you really think that if we traded Nesmith to GSW, that they would "develop" him into a player or do you think he would sit on the bench there too?
Some players do seem to be stuck in a sort of limbo. Too good to be G-League but not good enough for regular NBA playing time (at least on teams trying to win). Nesmith is that but he just needs to get better if he wants to earn NBA playing time. Or I guess get traded to a team like OKC or HOU. I don't think it would be any different on GSW or SAN.