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

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.