> Also, given that India semi-recently blocked some entertainment apps on national security grounds (let's assume the stated intent for the sake of the argument), I wonder if anyone in the Indian government is alarmed that U.S. megacorps hold IMO much more precious keys to the financial security of their 1.4B population?

Most of these payment systems are just a wrapper around GOI's UPI (which is one of the best things to come out of India in the past decade)[1]. The money remains in banks throughout the transaction. Additionally, very little of people's money is stored in digital wallets and that too for a short period of time (anecdotal). However, yes it should be a cause of concern for any country when foreign firms are tightly involved in its economic sector, and we cannot discount interventions on basis of internal security and geopolitical situations.

[1] https://en.wikipedia.org/wiki/Unified_Payments_Interface