I mean in your example it wasnt the software that was bought, but the website no? If most of the project revolves around a single site that uses the software then this is inevitable.

If nobody is willing to take over the maintenance of a fork, then thats just what happens. Its not like there is a deadline to this tho. Anyone could still just fork the codebase of a project that went to shit 10 years ago and start maintaining it again.

If people are willing to work on a project for free thats cool, but without any donations it will often be too much. To get donations you need a central, well known platform that represents the entire project. This is hard to do for a fork, because potential donators would initially have to trust complete strangers to not just abandon the project again right away.