I have seen a couple gyms (Ys) remodeled where stalls have replaced open showers. In one place the response by most of us was to simply not pull the curtain. What's the point? In the other the place lost the open culture.
I have often noted if you are a guy who will use a gym shower you are ready to use any shower. If you are a non shower guy you aren't likely to use any shower option including a stall if you still have to get undressed in a locker room or leave your towel on a hook. These non shower guys mostly will avoid any getting undressed.
This makes the shower stall phenomenon all the more ridiculous. We are getting stalls built to suit the sensibilities of people who mostly don't shower and never will.
To your point in recent years I attended two gyms while out of town that had open shower heads on one wall opposite stalls on the other. Over a dozen or so visits I saw two guys use the stalls.
I agree with everything you've said, and have observed the same thing. I've been a pretty regular gym goer for about 20 yrs, since my mid-20's. I almost always go in the morning before work, which means I'm there when lots of guys use the showers. Two of the gyms I went to for many years had translucent partitions between the showers, but no curtains or doors. The showers were pretty open and any guy using them was naked in front of everyone else. They also each had one private shower with a shower curtain. I would say literally 99% of the guys would always pick the open showers, and would only use the private shower when all the others were in use.
My current gym had this same set up, although the handicap shower in the back had no curtain, so there was no private shower at all. This didn't seem to deter anyone, as the guys at this gym always seemed to be very casual about locker room nudity, even the young guys. About a year and a half ago, the put curtains on all the showers. As AdamFin asks, who are these private showers for? There was no demand for them. Interestingly, the shower curtains all came down a few months later, with the exception of the handicap stall at the end, which still has a shower curtain. So now the guys can choose between the open showers and the one private shower. Guess what? No one ever chooses the private shower, even the young guys. The only time it's used is if all the other showers are taken, and even then, I don't think anyone bothers to close the curtain.
Even though I'm bi (leaning on the gay side of things), I don't see the locker room as a sexual place. I enjoy the ability to be freely naked with other naked guys. It's hard to explain, but perhaps it's a form of male bonding. To me, it's guy time and I enjoy it.