I think bus factor would be a lot easier to cope with than a slowly progressing, semi-abandoned project and a White Knight saviour.

In a complete loss of a sole maintainer, then it should be possible to fork and continue a project. That does require a number of things, not least a reliable person who understands the codebase and is willing to undertake it. Then the distros need to approve and change potentially thousands of packages that rely upon the project as a dependency.

Maybe, before a library or any software gets accepted into a distro, that distro does more due diligence to ensure it's a sustainable project and meets requirements like a solid ownership?

The inherited debt from existing projects would be massive, and perhaps this is largely covered already - I've never tried to get a distro to accept my software.

Nothing I've seen would completely avoid risk. Blackmail upon an existing developer is not impossible to imagine. Even in this case, perhaps the new developer in xz started with pure intentions and they got personally compromised later? (I don't seriously think that is the case here though - this feels very much state sponsored and very well planned)

It's good we're asking these questions. None of them are new, but the importance is ever increasing.