As much as I enjoy watching Naser run - her stride length is superb for her height, and her running action is perfect - she is clearly doped. You only have to familiarise yourself with her backstory to know she was probably a victim of doping when a teenager - and I use the word victim intentionally, because she moved overseas with her coach when she was mid-teens without her parents.
Also note the obvious: it matters not how often someone competes on the circuit, they can still be doped. Marion Jones anyone?
Back on topic. Yes, those records will be broken at some point. Will SML & Mu be the ones? Well it's a bit of a paradox, because as much as I think Kersee would be the one to take women to those WRs (because of his medicine cabinet expertise...) you also need to run, race and get a rhythm in those events. You can't just step on a track twice a year and run a 47.5 400m. You need to be in competition mode, at competition pace, and it's not easy (possible?) with so few meets a year. And the 800m is about tactics. Mu may have been able to front run her way to golds in the past, but there are more women willing to mess up a race pattern now (e.g. Moraa). Perhaps Mu will be able to front-run unchallenged at a US meet, but in a DL meet, or Championships, she is potentially going to be impeded, or blocked, or prevented from having a free run (if the other women are smart enough).
Who else has a chance, or could have broken the 400m?
The way she finished in Doha 2019, SMU clearly could have gone quicker had she timed her kick better. She had far too much left in the tank. I believe she could have gone 48.0 in that race just with an earlier kick alone. But that's still 0.40 off the WR. To run that, she needed to go out harder. She looked like she cruised that first 200m, hence being able to finish so strongly. Perhaps a slightly faster opening 200, and kicking earlier, and it might have been a 47.8-47.9. But still not a WR.
Naser's race plan was more akin to Koch's, which was set out at a super fast pace. Someone needs to do that. She was probably in 22.2 form over 200m in reality (ignore her 22.5 PB from that year..) so she was fast....but again, not as fast as Koch.
Bol is physically in the SMU category, and with her 49.2 indoors, can clearly go well into the 48s. Perhaps if she were to drop the hurdles and focus on the flat, doing more 100/200s too, she could be top 5 all time in the 400m flat. But she will never have the sub 22 speed needed.
That seems to be the bottom line. Sub 22 speed. SMU had it, but still couldn't break the WR. Give her stronger 'roids and she could have. Whether she was/is clean or not, if she were not, she clearly wouldn't be on the same regime or scale as Koch was. SML will never break the 400m WR unless she is doped. Ergo, if she breaks it....