> If Windows didn't have to support a gigantic universe of old-yet-critical software

Interest point, but not exact.

MS have close to monopoly state and is very close to formal margins, where regulators must issue regulative measures.

As I know, nearly all companies achieved so huge share of US market, got warning from regulators, and most immediately hit brakes to limit their share and avoid measures. Examples from past are IBM, Commodore/Atari, etc.

But what interest, using some obvious things, like just strip API to limit share, considered by regulators as offense, so subject must not do direct things to limit his product, and only could slow innovations.