The flip side of this is that every attempt at advancing the Windows UI framework story beyond win32/MFC and WPF has failed and the platform itself is steeped neck deep in technical debt.

You can't blame this all on backwards compatibility. Microsoft can't just stick to one successor technology and keeps obsoleting every attempt after a few years. With that approach, of course no one is going to move over.

That’s one issue, but another is that they don’t make into successors full replacements. They’ve stuck to WinAppSDK aka WinUI for a while now, but it’s missing such basic elements as a datagrid/tableview which makes it difficult to take seriously as a desktop UI framework.

Also, they didn't even migrate their own internal dialogs with each replacement. You can still uncover dialog styles all the way back to Windows 95-style if you poke around Windows 11 long enough. Whenever they came out with yet another successor, they haphazardly migrated a handful of P1 components and then called it a day.

Snow leopard is a unix based os. There is a ton of software that can still install on it and work fine.

Easy to say - much, much harder in practice.