We've adopted a framework as a small team and it has nothing to do with onboarding new people. We adopted it became clear that if we stuck with vanilla JS, we'd end up re-building something like jQuery to keep our code and abstractions manageable. And then we'd build something like React, while doing a much poorer job of it. Once your app has some "click X and A, B and C should update" logic you quickly descend into a tangle of tracking dependencies by hand.