If PR is good, maintainer refunds you ;)
I noticed the same thing in communication. Communication is now so frictionless, that almost all the communication I receive is low quality. If it cost more to communicate, the quality would increase.
But the value of low quality communication is not zero: it is actively harmful, because it eats your time.
But, crucially, if accepted, the contributor gets to draw 5€ from the repository’s fund of failed PRs (if it is there), so that first bona fide contributors are incentiviced to contribute. Nobody gets to profit from failed PRs except successful new contributors. Virtuous cycle, does not appeal to the individual self-interest of repo maintainers.
One thing I am unsure of is whether fly-by AI contributions are typically made with for-free AI or there's already a hidden cost to them. This expected cost of machine-driven contribution is a factor to take into account when coming up with the upside/downside of first PR.
PS. this is a Gedankenexperiment, I am not sure what introducing monetary rewards / penalties would do to the social dynamics, but trying with small amounts may teach us something.