Perhaps. Certainly source visibility makes life easier as a dev who has to live with these frameworks on a daily basis. That said, I think frameworks should strive to have a wide and deep enough set of widgets that in most cases, devs won't need to resort to custom implementations to do what they need to.

It also doesn't fix problems inherent to declarative UI, like readability breaking down easily and certain things that would be a cinch in an imperative framework being extremely awkward to implement.