Not everyone who uploads an open source project to some random cloud hosting service (i.e. Github) is qualified or wants to be a maintainer. Remember that being a maintainer is equivalent to project management. A lot of people publishing open source code don't seem to care about this and just want to write interesting programs. And that's fine. I personally have sent patches to lots of people who just never responded and their activity trailed off. Doesn't bother me in the slightest, that's what the "fork" button is for.
You also say "legal right" but it's incredibly vague what this is supposed to mean. Nobody cares about this stuff unless the project reaches a certain level of popularity. So once you get over 1000 github stars, should github force you to sign a contract saying you'll respond to emails in a timely manner or they take away your stars? I don't think I need to explain why that doesn't make any sense.
No, no, no - none of this is about legality. Just sometimes people bring it up as a framework for determining responsibilities, so I addressed it ahead of time. I don't think (legal requirements) == (responsibilities). The sets may have some overlap but clearly they are not identical.
Instead, I am just saying that if I upload a "cool tool" to github, and I notice that it has become super popular, and I don't have time/interest/inclination to maintain it --- I would personally feel a RESPONSIBILITY, and probably social obligation (not a legal obligation) to either put a note saying "this is not actively maintained, please find an up-to-date fork. user/cooltool seems to have community support but I can't personally endorse it." or simply make some other people additional/replacement maintainers for the project which I'm not interested in maintaining anymore.
Maybe you do, I'm sure people would appreciate that, but it's not necessary. The nature of these things is that the users are already "following the herd" to a degree. So if the maintainer becomes inactive and a fork picks up the interest, they will just move to that. No ceremony required.
You also say "legal right" but it's incredibly vague what this is supposed to mean. Nobody cares about this stuff unless the project reaches a certain level of popularity. So once you get over 1000 github stars, should github force you to sign a contract saying you'll respond to emails in a timely manner or they take away your stars? I don't think I need to explain why that doesn't make any sense.