> That is, even if child components got -rerendered (in react terms), there might not be a DOM update at all (which is the slow, costly operation).

Crawling a large React tree's render stack is also a costly operation. Not as much as DOM manipulation but it is still costly, and React is created in such a way that it requires you to include everything in that stack, even if it never changes. Which is one reason we have Portals.