This is part of the explanation but it's clearly not the whole explanation. The other part is why Windows has gone through half a dozen different UI toolkits wh

This is part of the explanation but it's clearly not the whole explanation. The other part is why Windows has gone through half a dozen different UI toolkits while macOS is recognizably the same with some re-skins since 2001. There's no backwards compatibility to be broken if you get the UI right the first time, and there's no excuse for introducing a whole new UI model with every release. It makes the whole OS feel like a gigantic pile of technical debt, and gives the appearance that engineering simply moves on and greenfields a new UI system when the old one becomes unmaintainable.