There should be a "then take over maintenance" option. A good maintainer also steps back and hands over the project. That is less troublesome for the community I think this is a good practice.
But ignoring good practice: a unpaid maintainer owns nothing to anyone.
1. Someone writes a library
2. Library becomes insanely popular
3. Users receive value out of library
4. One day, users stop receiving value from library
Is the author not responsible for the value generated by this library? At what point does the author become required to continue providing value?
"Just fork it" is an excellent response to (4), anything else is what entitlement looks like.