It means that the project depends on the whims or life events of a single person - a single point of failure if you like: critical patches may not be merged, the maintainer might turn malicious etc. More people on the project means more eyeballs on the code. This seems self-evident to me (especially if the project is core in ones tech stack) so I'm really surprised at the "what's the argument for?". Am I missing something?

(I don't see how you can cite Vue as an example; Evan is the main contributor to Vue, but he is certainly not the only one.)