I think this will somewhat police itself, with Wemby and Fox (and possibly Harper) standing in Castle's way from ascending to pseudo-star status. I think by simple virtue of the pecking order of our team, it's going to force Castle to develop the things that you (and we all) feel he needs to. Or at least I hope so. If not, then it will be because he has Diva mentality and he'll just go down as "what could have been" - but I'd bet against that based on what we've seen so far.
There is no doubt Castle has areas he needs to improve on (almost all second year players do), but there's also no reason to be lower on Castle than any other second (or even third) year player. Like I said to mo, IMO there is reason to be as excited about Castle's promise as any other player drafted in the last 3 years save for the very elite players (which I would call Wemby, Amen, Paolo and probably JDub. Brandon Miller and Chet are guys in the next tier who you can make a reasonable case for having more promise than Castle. But there is no one else in the last 3 drafts that I'd take over Steph).
I can see the case for Castle, but I disagree that Harper, who has yet to even be drafted, needs to "take a step back". He's about to be the #2 pick and allegedly he's been the #1 pick in a lot of other years. If he needs to take a step back and learn how to be a role player... then either he isn't worth the #2 pick, or he's not the strong prospect he's made out to be. If we were taking Flagg, I don't think a reasonable person would say "Cooper needs to learn to take a step back first". That just sounds like Pop meme "he needs to get over himself". If Harper is as good as advertised, he should be able to step right in and be a dude from the get go *on a winning team*. He'll still be behind Wemby and Fox, but the same way JDub came in as a rookie and contributed to a .500 (effectively) OKC team as a rookie, or Chet for a Top Seeded OKC team as a "rookie", Harper should be able to do the same in my humble opinion.
I think the Spurs are very fortunate that two very promising young guys, Castle and Harper, have the luxury of coming in behind Wemby and Fox. Unlike Wemby, they won't be expected to come in right away and be the #1 guy or even the #2 guy. Pressure is reduced and they can develop on a more natural pace, but I disagree that we needed someone like Durant to even further reduce the pressure on them. Durant would have been nice (at least for one year) because he is a good player and he fit, but not so much because we need him in order to make sure Castle and Harper develop "the right way".