I mean sure it's not impossible if you are willing to spend hundreds of billions of dollars to tunnel additional subway lines all over the place.

When they were built, these subway systems obviously were provisioned over expected capacity. But obviously, cities grow and nobody has a crystal ball to know what the population of a city will be 50 years from now.

The thing about subways is that adding significantly more capacity on an existing line isn't really possible if you are already running the trains as close as possible together as safety allows, which is often the case at rush hour. It's not like buses where you can just add more to the schedule.