I rode with a slammed -17° stem for 20 years, no problems at all, but I worked on back fitness by stretching and at the gym. The usual road fit isn't about looks, it's about long distance comfort, much more than it's about being aero.

My back did start to hurt as I got older - off the bike that is. MRI said lumber stenosis, arthritic facets, thin discs. On the bike, it never hurt, I think because I was fit and kept a flat back. If your back's flat there's no problem. See: Riding Position Discovery

Last year I got the first bike fitting of my life. The fitter said I had too much hip angle and moved me up to a 6° stem. He also had me put on a lower set of clip-ons to make up for it. My thighs almost touch my ribs when I'm on them. I don't really notice the difference of the smaller hip angle. I rode a 15 hour 400k with the lower stem, no back pain, in my early 60s though. I've stopped doing anything over a double at 74. I'm not unusual. Other riders in my group have had back pain helped by increasing drop. There's a good article on hip angle here: https://coachendurancesports.com/bik...nt-is-useless/
The author stresses hamstring flexibility. I can put my knuckles on the floor, but not my palms. I think you're correct about back pain and fitness level, but rather than change bike fit, it's better in the long run to increase fitness and keep the comfortable fit. Getting older doesn't suck as much if one tries to stay fit.

It's kinda like this: you hear old folks grumble about how it's become uncomfortable to bend over and pick stuff up or to get down on their knees. But does that mean they shouldn't do these things? No, on the contrary it means they need to do a lot more of that or pretty soon they won't be able to do it at all.