> what if Microsoft would give their engineers a few decades of time just to refactor their existing systems

They wouldn't, even if they didn't care about shipping new features. The technical complexity inside Windows/Office/old core MS offerings arises only in small part from ordinary tech debt--that is, things fixable without changing observable behavior. The lion's share of that complexity is because of Microsoft's priority to avoid breaking compatibility (backwards, or with hardware/software combos that work) at all costs.

That means that a refactor-only decade would achieve little unless that compatibility priority were changed.