If the project is valuable, someone will step in to maintain it.

If nobody does, its value didn't justify the cost of maintenance.

Perhaps all the people using it value it enough that they might pay (if they had no other choice to keep it), but might not be able to maintain it.

In that situation, it's valued but it only gets maintained if someone offers to do the maintenance for pay.

If the current maintainer is stepping down from the project if they aren't paid, but willing to continue for pay, it makes sense for them to let everyone else know they are willing to continue if paid, doesn't it?

If your finances are not in order, you can't be worrying about sustainability of some library or tool, or the industry for that matter. The industry is made out of people. If there are individuals who have to slave away to maintain an important tool or library, it's not going to be sustainable either.