For some of them, the level of usage just rapidly exceeded the authors' expectations.

Then people want you to add features, complain that it doesn't do something it doesn't claim to do, or complain that it doesn't work properly when used with other, independent software X.

And they really don't like the 'submit a PR' (stated more eloquently, of course) response usually.

I have one open source project that has gained some actual user base. I've been very open to accepting/merging PRs. I have a full-time job, a part-time job (few hours, by choice not necessity), other hobbies, etc. I don't spend a ton of time maintaining my free and open source project (its in a stable environment and doesn't really require maintenance to keep its core functionality, and won't really need to change unless the stack its built on makes a lot of breaking api changes).

I think the only thing that kinda sucks is namespace littering. Take PyPI -

User A submits a package called "DoAwesomeness"
Other projects use this as a dependency
User A abandons the project
User B forks the project and calls it "DoMoreAwesomeness"
Other projects have to manually update

In simplest terms, anyway.

There's probably not a great way to handle it outside of petitions to the package manager maintainers or something, but then what qualifies as a 'dead' project? Which fork gets to take over?