People always complain about this, yet nobody contributes: http://who-t.blogspot.com/2019/10/libinputs-bus-factor-is-1....
Imagine if all doctors and surgeons were volunteers; that’s the situation you have now with a lot of OSS.
It's a concept that basically boils down to "people would not be smart enough to understand the code unaided". This is plainly wrong. I speak from experience of diving into some horrible code base.
It also assumes that reproducing the functionality from scratch would be hard. Looking at all the reimplementations of A where A is one of unix, X, web browser, network stack, SQL database and other complex techno, it is also clearly not true.
I also don't see bus factor the same way you do. People can be smart enough to understand the code, but unless someone actually goes and actively takes up code maintenance the project will slowly fade and disappear (for example the synaptics driver that libinput replaced https://github.com/freedesktop/xorg-xf86-input-synaptics/com...).