1.) Sochan's utility makes him invaluable off the bench
When looking at the Spurs' projected starting lineup of Jones, Devin Vassell, Johnson, Wembanyama, and Collins, the two players most often suggested to come off the bench are Jones and Johnson. However, both players fill specific roles in the starting lineup. Jones is the team's best playmaker, and the Spurs are unlikely to have Wembanyama start his career playing in experimental lineups that don't include a traditional point guard.
Meanwhile, Johnson is a key part of the team's offensive attack, along with Vassell. Benching Johnson in favor of Sochan, who is primarily a defensive player at this point, wouldn't optimize either player. Instead, the Spurs could simply keep Johnson in the role he is best suited for—starting at small forward—and have Sochan play as the team's third big man.
That would allow him to sub in and play with either big while staying at power forward. Better still, he would still be able to play a large role. After all, Wembanyama and Collins may only average around 30 and 24 minutes per game, respectively.
That would allow Sochan to still average around 30 minutes, close games, and have the same impact he otherwise would. On the other hand, Jones and Johnson likely wouldn't have the same impact off the bench. Therefore, Sochan coming off the bench next season would be beneficial to everyone.