I don't see either hit-by-a-bus or slow-decline scenarios as worrisome. There have been times when other people's trees have been used widely. I think we'd see one of those get bumped up, in a pretty seamless way. git makes this easy.
Linus's tree has maintained its position as "mainline" because everyone thinks of it as such. If it stopped moving in the right direction for whatever reason then people would start using another tree as mainline in a way similar to flocking behavior in birds. There would be no "choice" as a group -- all of a sudden you'd see most other people treating tree X as mainline, and so you would too.