Both these systems operate from around 5am till midnight (approx). NYC Subway operates 24 hours. These few hours downtime, make a huge difference with respect to maintainability and upkeep. These 4 hours each day adds up to ~60 full days of maintenance window.
Maybe NYC can stop the late-night services for a year or so and I am sure better brains than mine have considered this.
On a separate note, it might help to note that usage of NYC Subway has increased by 80% over the last ~20 years (1). It will be interesting to see the same trend for Moscow and Tokyo subways.
NewYorker here and Subway rider here.
(1)https://www.nytimes.com/interactive/2017/06/28/nyregion/subw...