While I agree this is great, it is not really related to and no excuse for the inconsistent mess that Windows' UI has been ever since the Windows 8 Metro disaster. Win32 UI (think Windows 2000) was perfectly line.

If I want to run an old program I'd rather do it in a VM and have a nice, lean, consistent, sensible OS.

You're a technical person. Grandpa would go: "VM? What's that???".

The answer to this is simpler than it seems: instead of dedicating your compatibility efforts toward a monolithic OS that runs everything, dedicate them toward make the VM/shim layer so transparent “Grandpa” doesn’t have to know it’s an implementation detail.

In contrast with the earlier comment about how poorly Apple fares on backcompat: they have twice (thrice, actually, counting 68k->PPC) successfully made breaking platform changes where old software kept working with no required intervention by users or even knowledge it was happening.

Not everyone has been happy with Apple’s deprecation strategy, but there’s no technical reason MS couldn’t use their subsystem prowess to gradually move old technologies out of the core OS into sandboxed environments.

The reason they don’t is partly cultural—the compatibility at all costs trope is real—but I think it’s mostly a combination of lack of motivation and a recognition that the ecosystem produces a lot of abandonware that would quickly show seams.

This is what I had in mind. Something along the lines of VMware Unity.

Shipping an OS where some bits of UI date back to Windows 3.1 is never going to lead to a seamless experience.

Tell that to a bookeeper who runs aincent software the company has licensed decade ago or to the factory worker who must use a CAD software for a particular CNC machine to prepare the work files.

So yeah, legit but niche use case. Thankfully I don't need to worry about it so don't give it a second thought.

windows's long tail is niche use cases