The bad defense is in large part a product of players just being young, but I also think there are likely more legit bad defenders than there are decent ones, let alone good ones.
By position:
PG: Jones is basically the only consistent bright spot on defense. By RAPTOR, he's a top-20 player at his position, and on the defensive end, he's 13 on a list that includes many bigger guards and wings who moonlight at PG sometimes. The Spurs could improve at the position, but he would totally be able to fit as a cog in a great defense just as Parker fit into the Spurs of old despite his personal limitations.
SG: Vassell is sort of the same in that I think he'd play well with a strong defensive core, but he is more of a product of his surroundings than Jones, who's arguably the only thing keeping the Spurs from absolutely worst. I do worry about him picking up some bad habits, but he has good tools.
Langford is getting a ton of run, and while he's grading out well enough defensively, I don't see how he's a keeper. Maybe on a cheap contract coming off the bench? As a starter, I just think he leaves too much on the table.
Richardson like a few Spurs looks like he's defending well while grading out horribly. On the continuum with Jones and Vassell, Richardson would probably contribute with a strong defensive unit, but he's more in like "He can hold down a seventh man if necessary" category. I like Josh a lot and wouldn't mind him on the team long team if they are somehow able to make a quick pivot after the draft. But it's starting to be too many names who'd be great surrounded by a core and not enough of that core itself.
Branham. Bad as advertised. I think he grades out as literally the worst defender in the league. Yikes. Hopefully the kid can clean that up.
SF: Keldon playing SF has been an unmitigated disaster. He's having his worst year, and he's much worse than even the average Spur on that end. I think you can make a good argument that losing the weight has helped his offense, but the insistence that Johnson's ineffectiveness came down to not playing his "natural position" is currently unsupported. He's seen a decline in net-impact, which is insane considering his improved scoring.
McDermott is only not the worst-defending Spur in terms of RAPTOR because Branham exists. He and Johnson trade poor statistical rankings. His is easily the Spurs' weakest position.
PF: Sochan is an ineffective defender who is trying way too hard to be one of those "mind games" guys who honestly went the way of the dinosaurs like 10 years ago. He obviously has potential, but I'm not sure players usually rise to become elite defenders after starting out as bad defenders. This contrasts with someone like Tari Eason who's actually already a good defender.
Stan is grading out to be a bad defender, but he's also the 10th man and contributing a lot on offense to where he's one of the most positive players on the team. It'd be unacceptable long term, but he's not the team's problem right now.
KBD: He's another entry in the "good piece on already great defensive team but not a foundational player to such a defense" ledger. For a team defending as badly as it does, Keita's almost-neutral rating actually suggests he's a positive contributor. But he's a really streaky offensive player who nets out as pretty bad on that end despite the flashes, so the team is probably not missing his minutes.
Roby I feel didn't get much of a chance this season, though he's literally never been more than a below-average defender despite having a profile of someone who'd be good on that end.
C: Poeltl is good, but he's always lacked the top gear to be a truly elite defender. He's pretty mobile for his position and protects the rim smartly. His man defense has always been meh, and his boxing out has been spotty against any decent center. He's not hurting the Spurs, but I do think he has to play better.
I really don't like Collins. He's a goon who tries to injure people. Like Sochan, he seems to have an antiquated idea of what good defense looks like and isn't even effective at that style anyway. Every once in a while, I'll look up to see him have an incredible statline. So maybe he has good trade value somewhere. I'd explore than if I were the Spurs. He has some potential as a former 10th-overall pick and is mostly healthy now. But I can't really root for him.
Bassey's been okay. He's young and dumb. but he's alethic and seems to take some pride in blocking shots. I wouldn't mind seeing him.
Dieng is done.
It's easy to say that the bad defense doesn't matter because the team is tanking anyway. But it's very possible the team is going to have excise a fair bit of the roster, including multiple young players who had potential, if they're going to figure it out on that end. The level of bad defense we're currently seeing is unacceptable to my mind. While I think it's possible to learn to play a defensive role after not being a good defender, I don't think one can take a club full of bad defenders and get anything positive out of it on that end. They're going to have to strongly limit the number of bad players in their rotation, and right now they're way, way above critical mass.
I'd also question if the defensive concepts Pop and his staff are teaching are really ideal for the modern NBA. I haven't looked too much into what schemes the Spurs are running and what principles they're employing on that side of the floor. But it's weird that even with so much turnover, the Spurs still seem to allow so many wide-open threes. It's possible that they do that way more by design than some may assume. I could see an old scheme allowing those shots if it's from a time when three-point shooting percentages where much lower and hard closeouts were way more effective in disrupting opposing shooters. Even if there is a schematic compromise that the Spurs are unwisely accepting, the personnel is the main culprit for the current nadir, in my eyes.