I agree with
objective that Lyles is basically a gate-keeper for a young PF. Pop probably knows and is comfortable with what Lyles brings, but it's not to the point where he can't be unseated. We could easily see a situation like Walker/Beli were Pop keeps giving the young PF chances to unseat the vet but always keeps one foot in the vet's corner ready to switch back at a moment's notice.
I really think there will be some type of trade that could affect where the Spurs are drafting. My gut tells me that at least one of Gay/DeRozan/Lyles will be gone next season, leaving room for Sam or a drafted PF. But just as with you, I could definitely be wrong.
I'm suggesting this rotation:
White, Johnson/Walker, DeRozan, Lyles, Aldridge
Mills, Murray, Walker/Johnson, Gay, Poeltl
Then Sam would be the 11th man who gets minutes from all three front-court positions (with the loser of Walker/Johnson getting some minutes from Murray and Mills). I don't know if Pop feels like Sam can play the three (though remember he said he thought Metu could, so it's possible). I do think he will consider playing Sam and Gay as the forwards without specific thought to who is playing what and see if it works. Sure, that's two PFs, but Pop routinely played two centers together back when he had three rotation-caliber guys.