In my experience from a anecdotal statistical point of view it’s kind of a non-issue. In all the thousands and thousands of times I’ve used a public toilet stall handicap or not I have only ever once encountered a handicap person waiting for a stall once. But so were 5 other people. So that was like 0.01% of the time and I’m not sure it counts anyway as the man in question graciously waited his turn after other men who’s could have use the other stall that was occupied. I think he was right to decline to cut inline because a handicap doesn’t necessarily mean you get to skip the queue.
On top of that in terms of infrastructure cost the two are vey different. A parking space is several orders of magnitude cheaper than a bathroom stall. That’s the reason a building will have hundreds of parking spots but only 2 or 3 toilet stalls. Cost and usage patterns are completely different therefore so are the rules. Reserving a stall for 10 minutes of use is VERY different than reserving a car space for 2 hours of use. If accessible toilet stalls were meant to be reserved, they would be, just like parking spaces. So don’t over think it and just do your business.