The least surprising thing about the xz vulnerability is that it happened to a widely used project after a maintainer hand-off. We've seen exactly the same thing repeatedly in npm, pypi, browser extensions, and other code marketplaces.
Humans don't last forever. Hand-off is inevitable. And I've long held that because that is true, the size of the group of maintainers is an important security characteristic.
Small projects create big risks.
Sustainability is a security concern.