While some elements of new UI definitely require improvements to OS underpinnings (e.g. the idea of adding gradient blurs everywhere), the vast majority do not.

While some elements of new UI definitely require improvements to OS underpinnings (e.g. the idea of adding gradient blurs everywhere), the vast majority do not. Why can’t we have OSes layered better so that the “core” can evolve but these “design”-of-the-week changes can remain compartmentalized and potentially ignored from one release to the next? To a large extent, nothing ought to break if you simply allow users to choose from any of the “looks and feels” of the past few years while updating the lower-level OS to be the latest. It also means you could have different security policies, e.g. prefer to auto-update more at the lower layers and auto-update less or never at the GUI scale. And, any “correct” application is somewhat layout-agnostic anyway: technically with various Accessibility settings you can already change the look and feel and behavior a lot, and apps will not work quite right if they don’t respect these settings. There is not much difference between those changes and any Apple “design” from one year to the next.