PCCs were also in the pre-accessibility era which made "one size fits all" cars much easier to produce, now transit systems consider all their riders which means much tighter tolerances which makes it pretty impossible to use a single design. Now we expect platforms and floors to line up perfectly and gap free. Also to make the most of a specific size of tunnel custom built cars are the only way to go, the current green line cars are much larger and come within an inch of the walls at times but are designed as such.
You're right that ADA has made it harder, but let's not pretend US transit systems do a great job at this. Remember how much money and time the MBTA spent to raise every green line platform by 3 inches, and we still don't have true level boarding? Or how the Silver Line platforms were built at standard curb height rather than 2 inches higher to allow for true level boarding?
And dont get me started on Amtrak ordering like 200 new high level trains to run in California and Michigan where all the platforms are low-level...(they get delivered next year).
Look, I fully agree you can't one-size fits all model for every transit agency. But thats not the same as saying every order should be 100% custom - especially on the regional rail side, where things ARE pretty damn standardized around the country.
It's it funny how all the competing private rail freight lines have managed to use standard rail cars?