Or the network finds consensus on an algorithm change where the block subsidy keeps going, thereby continuing to pay the miners. The 21 million bitcoin cap isn’t as immutable as people claim. It’s just that most Bitcoin stakeholders agree to keep it there, for now.
That Bitcoin is some immutable “backed by math” financial system has been a charade all along. It’s very much backed by people, and proof-of-work is a Rube Goldberg machine used to distract the tech-literate.