He is definitely stunting them to an extent. Not only does he take away ball handling and scoring opportunities from every young player, he simply forces them to feed him for an offense-killing, inefficient score attempt every single time. He makes Murray and White's jobs harder by being ass on defense, forcing them to help. He sets the mood of the team and constantly gives up on plays, especially on defense, whilst taking every last minute shot and being ass at it. How is any of this not demoralizing and damaging to the rest of the players?
Yes, Murray wouldn't instantly learn to shoot 3's, but he'd get a lot more chances to try them. God knows I'd much rather have DJ try and miss a thousand 3s for his own development than watch DeMar, a known quan y and developed player, taking yet another contested 20ft shot. LMA is also a problem for the team; not only has his defense been very bad this season as well as DD's, his heavily declining athleticism impedes him from running the naturally fast-paced offense that the young players prefer. You saw DJ at the start of the season, running on transition and initiating fast offense... Only to have to wait for LA's corpse to drag up the court and demand the ball. And you know there's orders from above to give it to him.
DeMar and LMA are inequivocally stunting the Spurs. Any other average replacement players, simply by virtue of not being "stars" that need their touches, would make the team cohesiveness better. Would the team overall be better? Maybe not, we'd probably lose more games. But it would be watchable, instead of the trash product we're currently rolling out.