Almost all franchise players end up on teams who would have taken them higher. The Knicks loved Curry and probably would have taken him in the top three (yes, I know GS drafted him). MKE loved Giannis before anyone else and considered him untouchable instantly. SA when all covert ops when scouting Kawhi. Did these teams bend over backward to trade up to first-overall to guarantee they got their guy? No, and in the Knicks' case, that was a problem. But it wasn't a crap shoot for those teams either. It's not an accident that teams get reps for being great at drafting while others get the opposite.
When a player's future potential is a bigger driver of their value than their current production, they are prospects. No one is trading for Tatum because of what he can bring a team this year. Well, no one is comparing him to the first-overall for that reason anyway. Tatum is not a "sure thing"; there's not guarantee that he's going to improve a ton. You want to force that notion into your hypothetical, but I can't ride with you there. I've already allowed for you to suggest Boston would be willing to give Tatum and the Sac pick while Phoenix would only give up the first-overall. That was already a bridge too far, honestly.
But ignoring that, if the Spurs believe Ayton or Jackson is the next Towns or Davis, then the Sac pick simply isn't a factor in the decision, because the chance to grab a superstar is at least an order of magnitude beyond normal "value" discussions. Again, I'm not advocating for either of those guys or anyone in the draft over Tatum. But that's how they have to look at it. You just don't money-ball high draft picks, especially in basketball. If they don't have Tatum as having superstar potential, then there isn't enough future value to fill in the gap between him and a guy they love in the draft. (Obviously, this is assuming they have such a player in mind; if they don't like this draft, then they aren't going to be considering high picks in the first place.)