I agree with the problem but not the solution. Yes, paying people to get changes merged is an attack on projects. I've seen this first hand where all of a sudden random BS PRs emerged. But ...

"Open Source" is a license thing which requires public source.

Closed source is the absence of an OSI approved "open source" license and does not mean that the source is not public.

Open source projects normally cannot simply change their license as all contributors would have to be asked or you would have to remove all their contributions (or they were planning for this from the start and had all contributors sign a dual licensing form which gave usually a company the right to use the contribution in closed source, too.

If in my project something like this would happen, as a maintainer I would employ bots to throttle contributions. First time contributions to non-code files could get auto-closed with an appology comment.