They aren't asking for sponsorship on the tool they created. They expressed that they do not have interest in investing even more work to rewrite it in Rust, Go, or what have you; unless someone paid them to do it. And I think that is completely fair!
If someone has no inherent interest in doing something, is not othewise obligated to do it, it is not done as a favor to friends or something, paying that person to do the job anyway is a very accepted practice in our society. Almost all of our employers pay us to do things we might otherwise not do.
There is nothing wrong with extracting as much value as possible from a small effort. It just seems highly unlikely anyone would sponsor it so the request seems somewhat ridiculous.
If someone has no inherent interest in doing something, is not othewise obligated to do it, it is not done as a favor to friends or something, paying that person to do the job anyway is a very accepted practice in our society. Almost all of our employers pay us to do things we might otherwise not do.