Reason #2: Another in a line of homegrown stars
The Spurs have a history of drafting players and developing them into All-Stars. The most recent example was Murray last season, but other examples include Kawhi Leonard, Manu Ginobili, and Tony Parker. That may not have anything to do with Johnson per se, but it does fit with the narrative that the Spurs are star-makers, and that should help his chances of making this year's team.
Johnson probably isn't going to get in through fan voting, but he could be voted in by coaches, who'd probably have more perspective on the Spurs' track record and have also seen firsthand Johnson's improvements since being drafted. That alone should give him a shot at making the team. Even if he doesn't make it in through fan voting or coach voting, there are injury replacements, and commissioner Adam Silver has a history of making rising stars first-time all-stars.
That may be strategic since it gives the NBA new stars to market, but if Johnson continues to play as he has, he'd be deserving of the honor. That would be huge for both Johnson and the Spurs. Johnson's profile would dramatically increase after becoming a first-time all-star, while the Spurs would have a new star to try and build around.