Population is not very relevant metric here. What matters is not so much how many people actually live in Manhattan, but rather how many people are commuting into it every day. In fact, commuters double the island population every day, and exceed the number of residents in 1920 by 50%.
What happened with Manhattan was that tenements were replaced with office buildings, and these use more, not less commuting.
Manhattan commuters peaked in 1946. The train system was able handle that level of demand without falling apart. All of the messaging from MTA about increasing ridership causing delays is a disingenuous lie.
Considering the fact that much of the same infrastructure they used in 1946 is still in use today (with 74 more years of wear and tear) its not surprising that its falling apart. This would be true even if the MTA wasn't staffed by dishonest, incompetent political cronies. When you regularly do things like spend $600 million dollars on trains that don't work, its not hard to see how you end up billions in debt very quickly.
What happened with Manhattan was that tenements were replaced with office buildings, and these use more, not less commuting.