But that’s not what history shows us. The subway with the second most ridership in the US is WMATA. How did they get there? By going big with their plans in the 1960s. All the systems that thought small in that era (MARTA, BART, LA Metro) have much lower ridership to this day.

There is a good basis to this. Every new mile and station you add to a system compounds on the size of the system. The 11th station connects you to 10 places. The 51st station connects you to 50 places. Build small and you never get the critical mass needed to see widespread adoption.