My analogy is every Microsoft UI framework was almost completed in the sense of someone being almost pregnant.

A framework that has just one show-stopper missing feature or problem is... unusable. You can't embark on a large, complex application development journey if you even suspect that you'll be painted into a corner.

For example, many of WPF-derived frameworks had atrocious performance, with fundamental mistakes in their design that made them incompatible with list virtualization. It wasn't until they had to eat their own dogfood and use WPF for Visual Studio that they started fixing these issues.

Win UI 3 meanwhile dropped all support for HDR, wide-gamut, etc... going backwards to SDR sRGB only in an era where all mobile phone manufacturers were starting to standardise on OLED HDR displays. The logic behind this decision? Microsoft wanted a UI framework that is "mobile compatible"!