There's quite a few projects in linux where projects were started by individuals or groups that stopped working on those projects but were then taken over. A few examples:It's a risk and, again, we rely very heavily on altruism. If it wasn't for altruism, I don't think FOSS would be an option. We often don't even donate enough to cover the hours people invest in keeping the Jenga tower from collapsing.
OpenSSH was originally maintained by Tatu Ylonen, and grew, to then be maintained by a group in about 2001 and Tatu dipped out.
Xorg was created after the old XFree86 group applied a new licence to their programs which led to a group of maintainers splitting off to create Xorg.
Gimp was maintained by two guys, Spencer Kimball and Peter Mattis, who just stopped developing which led eventually to other guys eventually taking it over.
Cups printing system was developed by a single individual, Michael Sweet. The Apple company took it over employing Sweet, but stopped developing it in about 2020, so the Linux Foundation took it over with its OpenPrinting group.
There are heaps of examples, the above being the ones I'm familiar with.
It's worth noting that many maintainers are both altruistic, and paid to be maintainers. Companies like Red Hat, Canonical, Intel and Google employ linux maintainers who work on linux projects. Linux is now so integrated in the IT world, that any important software that is threatened with a loss of maintainer, is quite likely to be taken up by some alternative as in the above examples.
The issue I guess is going to be different in cases of different projects. Those that are large like the above, appear to be saved quite effectively. For smaller ones, the situation may be quite different. It may eventually depend on how much dependency there is on any one project.

