It makes perfect sense. to trade Murray even if Primo is going to be a star. In my opinion, calling the Spurs a treadmill team is a bad take that can only be rationally explained by being a person who hasn't watched the team. The Spurs in this scenario would have had basically the same record four years in a row despite collecting god knows how many firsts and young prospects due to the sheer growth of their talent. That's amazing. If that happens, they're in a much better long-term compe ive position than they would've been had they kept Murray. It's possible that Primo might not've become a star with Murray on the team in the same way Murray took advantage of DeRozan and then White being traded. But even assuming he did, the Spurs after this trade and the likely Poeltl follow-up are in a way better compe ive position than they would've been.
Had the Spurs kept Murray, their second-best player would've been a fringe All-Star getting ready to sign a max contract, and rather than having the ninth-overall pick, they'd be a lower-seed playoff team and pick around 20. Not only is that pick worse, but the rest of their future firsts are jeopardized by the assumption that the Primo/Murray duo would convey picks in the 20s for years to come. The way some fans see the Hawks picks the Spurs got, some of the oppose FOs would look at the Spurs' pick. So when it came time to make the trades they'd need to to ignite their core, they'd have much less buying power. Even to get a finishing piece, they'd run to risk at giving up what Minny did. Dicey to say the least.
Compare that to the scenario where Primo stars. First, Johnson and Vassell wouldn't be traded off. The Spurs are hoping to draft a franchise player, which is apparently what some in the FO Primo has the potential to be. That's multiple tiers above where they were said to have placed Murray. They dealt DeJounte because they didn't think he was a franchise player, If they did, they would have kept him and not entertained tanking. If Primo shows himself to be such a franchise player next year that he drags the roster to respectability, PATFO is going to view complimentary pieces like Vassell and Johnson as more important than they seem them now. It's possible they could get traded, but it would be in service of a win-now move for a second star. It would be for draft picks. They'd have their franchise player already. They'd have what you think they must tank for Wembenyama to get. I'm sure even in this scenario, they'd love to be able to draft him. But I'm confident they'd view Primo living up to their wildest dreams as the move important and validating aspect. They'd have a decent pick and literally others 10 first-rounders to play with. Having a franchise player in the hand is worth that much. Even if you miss out on the Lebron, you can still win a le with Wade.
You don't become as bad as you want them to and then just stop. You're talking about trading away anyone who looks good and rigging everyone's development if they show a sign of being "too good". You don't go from that to winning immediately. Even if the guy is a phenom like Yao or Garnett and makes a horrible team into a respectable playoff team, it's still due mostly to that player's sheer force of will trying to buoy a terrible roster only in the modern day of player empowerment and with guys not feeling pressured to risk their bodies for situations they don't want to be in.
Also, it's not like if they don't get the first-overall pick they'll all of the sudden get back the year of wasted development they pissed away for a literal dice roll.