There might not be robust competition between payment systems, but when I go to a store, I've got options to pay:

Cash, credit (discover is dieing, but amex/mc/visa compete a bit), debit (several networks and all US cards have to be on at least two), I've seen PayPal as an option ocassionally, some merchants take Zelle, FedNow could if a good UX comes around (I don't think many merchants want to give out their routing and account numbers, and it's tedious to input them into my banking app anyway). Some vendors take checks and deposit them later, some take checks and process them immediately, etc.

People respond to incentives though. If merchants charge the same regardless of payment method, I'm going to use a rewards card that costs them more. When they add a line item credit charge, I'll consider cash or debit.

I would be very surprised if Discover is dying. The whole point of Capital One buying them--well, a major point--was to have an alternative to MC/Visa on which they can run debit and credit cards, and so they'll be pushing an expansion of the Discover network rails pretty hard, I'd expect.