Ask HN: Is there a case for UI / layout libraries that ditch the past?

UI programming is hard and extremely hacky and fiddly. Virtually all the major UI libraries are deeply committed to backward compatibility. But surely the cost in terms of complexity, code, testing, effort and cognitive load is unbelievably high. Game changing browser technologies such as FlexBox have matured enough to be usable in current browsers. Such technologies make things dramatically more simple and understandable. Is there a case now for UI frameworks that ditch the past and choose the simplicity and practicality of latest-version-browsers-only?