fix this problem by make the rating value tied to some paid currency - a repo owner would have to pay for the PR, and that PR contributor will now have more currency than previously. In order to have said currency to pay, the repo owner would need to have contributed to another repo whose owner have currency.

The totality of someone's currency is their reputation.

Of course, now the decision becomes...who is the central currency issuer that creates it?

It's the StackExchange model! This has bootstrapping issues, is hard to break into the community, and risks creating moderator cliques.

This is called proof of stake