"First, you cannot fit a standard pram inside a traditional gapped cubicle either."
No shit Sherlock. You are posting on Mumsnet, we are very aware of the dimensions of toilets and prams.
"In that setup, a parent must leave their pram out in the communal sink area and lock themselves in a tiny stall, physically separating themselves from their baby."
Tell me you are male without using those words.
No... what happens in those cases is that the female person with the pram goes to the toilet with pram in the open doorway so that person has line of sight of their child.
"If the main advantage of a traditional single-sex space is that a mother is forced to use the toilet with the stall door wide open just to keep an eye on her child, that seems like an architectural failure.
The modern solution does not force parents to compromise their dignity. It provides properly sized, dedicated family/parent rooms specifically designed to accommodate prams behind a locked door, keeping the parent and child together in total privacy."
You are now using utopia to sell your solution. It is an issue with many toilets around the UK. There are very few parent rooms in the UK.
So.... where and when are all these parent rooms being built? In my local pub, where will that parent room be put? At the moment, there is a communal space in the female single sex toilet that provides at the very least privacy from male people.