Good further reading on this change is "Windowless controls are not magic"[1] from Raymond Chen, and the followup "Windows are not cheap objects"[2]. It's a fascinating tradeoff space. I think fine-grained windows worked reasonably well on limited hardware but did not scale to super-rich applications like web browsers, or a more sophisticated imaging model where widgets need to be composited with alpha-transparency.
[1]: https://devblogs.microsoft.com/oldnewthing/20050211-00/?p=36...
[2]: https://devblogs.microsoft.com/oldnewthing/20050315-00/?p=36...