Lots. You still ignored me actually showing that the Spurs don't tend to have player who improve. You using evidence about a different situation doesn't make your claim the default. You want to assume the Spurs draft the 28th- to 30th-best prospects every year and end up with the 12th- or whatever-best players. If that were true, those players would start off mediocre or bad before over the years getting better and then ascending to being among the best of their class. We don't actually tend to see that. Instead, during the Medium-Three era, we saw some of the supporting guys gain half-hearted recognition as they grew into their roles and them nothing for the past half-decade despite the team drafting numerous prospects and needing guys to improve.
Assuming there were only 10 top-10 MIP candidates every year (which is often not true as there were ties for 10th place), there have been 120 MIP spots including the 2009-2010 season. That means each team should have an average of four players make the lists. The Spurs have had four players do so. That's with the Spurs purposefully lowering the players' starting points by shunting them to Austin. That's with the Spurs supposedly having a reputation for developing players to aid their vote counts. That's with the Spurs actually outdrafting their draft slots on multiple occassions.
So what gives? Well one thing is that the Spurs hardly ever draft projects. When they do, the players bust. Sam, Lonnie, LJC, Richards, Metu -- Murray is the only success among them, and his rise hasn't been quick at all. He at least should get legit MIP consideration this year, but you could (and I have, exhaustingly) argue his overall impact was stagnant for year before this season. So no, the Spurs aren't drafting guys that they are building up into good players. Instead, they tend to draft good players and let them adjust. Hill, White, Green, Bertans, Forbes and Mills were good players (well not Bryn, but he could shoot) who needed to adjust to the game. Certainly they've improved, but Spurs fans often overstate how much by assuming they were bad players or overrating how good they are. That's why folks can say that Johnson has been a developmental success when at best the jury is out on him right now.
The Spurs have never, ever, been a team that created good players at a rate higher than the average team. They might have had the best shooting coach in the league at one point, but overall, their strength came from being able to identify player who could help them and fitting them in. They don't need to hang their heads that they haven't turned crap into gold. I fully accept they haven't had prime stock to draw from. But I'm not the one claiming the Spurs developmental team is special. The numbers just don't back that up. To show otherwise, you'd have to show players' growth compared to that of players on other teams. It's harder than just posting win-shares by draft class, I admit. But at least it would be part of the discussion and not just a non-sequitur.