> They tried to make it a sustainable project

> with the hope of sustaining the project with donations.. the amount has been too small to support the project

So shut it down, lock the repo, invite new maintainers indefinitely. It's only non-sustainable for the price the author is asking. It'll still be FOSS even if no longer maintainer by the original author, whether a new maintainer steps in or not.

And if they want to fork it an create a commercial alt, no problem - anyone can!

The problem arises, IMHO, when they develop (accept contributions) or propagate (lock-in) a FOSS project in good-will, then somehow leverage their position as a FOSS maintainer transform it to non-FOSS.