That's also why taking on Vue is a huge risk. The project has a high bus factor. If the main author burnt out and quit, the project is dead.
The #2 person contributes only about 1% compared to the #1 person.
Compare to React's same graph: https://github.com/facebook/react/graphs/contributors?from=2...
The #2 person is at about 50% of the #1 person.
Compare to Ember's same graph: https://github.com/emberjs/ember.js/graphs/contributors?from...
The #2 person is at 75% of the #1 person.