This is an argument we've been worrying about a lot recently: "What if Harper doesn't develop", and it applies to Castle and every future pick. The answer is the same: You trust the process and hope it doesn't happen, but if it does, you pivot. That's much easier to do when you aren't locking yourself into a single idea of what the team's "core" is going to be.
Also, folks like Castle and Harper becoming good rotation players but not stars is only a tragedy if you don't pay them accordingly. You don't max out a Harper who's not able to unseat Fox or Castle, but a) if he's a threat to get big offer sheets, he has trade value and b) that means the Spurs got years of quality sixth-man play for cheap, which given these are the Durant years we're talking about is really valuable. Often in these arguments of players having lesser value if traded later it gets lost that value is ultimately there to be used, not exchanged. Guys having less trade value after contributing to a winning team for years is completely fine. They aren't trying to get top dollar on every deal. They're trying to win games and rings.