I think the idea that a founder of a project is responsible to maintain it into infinity and beyond is missing the whole point of open-sourcing code. That founder is putting unfounded pressure on themselves rather than letting the project grow and evolve in an organic way. Much like a helicopter-parent that micro-manages their children's lives, rather than letting them make mistakes, learn, and mature into independent adults.
Obviously, there are a number of open-source project methods for handling this, forking being a common one that the community itself can apply. But I would think it would be beneficial to the project community to have an honest discussion about it an entertain offers to take the project over rather than breaking the community spirit of contribution that is core to the ideals of open-source programming. If there comes a time when no one is willing to step up, the community and more importantly repository providers, should offer a way to put a project into maintenance mode, and allow someone to come along and take over maintenance of code that has the time and willingness to do so.
This comment is not an effort to criticize the founders of the mentioned projects, but an open-ended question to the community at-large to ponder and reflect upon how we manage and grow the spirit of open-source, while taking care of the maintainers/contributors that help keep it going.