You just have to look at the peak ridership figures from 1949. They're running well under the capacity the system could handle 75 years ago. Faster trains aren't necessary.
It's definitely under capacity by that metric.
https://www.wnyc.org/story/subway-ridership-hits-65-year-hig...
The only issue is that then it arguably had a much larger tax base than it does now to fund it.