I imagine, it's just too much of a niche and practically not enforceable anyways.

You would need to somehow know that a web service is a using a modified version of your library, then you'd be able to demand those library changes to be open-sourced.

And well, just in general, covering all kinds of niche use-cases isn't terribly healthy for open-source licenses, because each modification is something that can be challenged in court and which might be incompatible with other licenses.
Ultimately, a library under such a specialty license would probably not see much use either. You could only really depend on it in AGPL applications. And at some point, you do have to ask yourself, if it's even useful to develop your library then.