That sounds...arbitrary and subject to politicisation.

What method wouldn't be? Putting it to a committee would have similar problems. Writing it into a law would be too static.

Especially because the first part of determining marketshare and market power is determining the market. New markets happen all the time, with old markets dying out. Twenty years ago, smartphones wouldn't have been a market. Fifteen years ago: streaming services.

Like going back to your "Intel with x86", what is the market? x86_64 CPUs? Sure they have monopoly power. All CPUs? Now they don't have as much because they're competing with ARM too. Mobile phone CPUs? I don't think there's been a new Intel phone since the Asus Zenfone 2 (2015).

The method of letting the markets decide would work, I think.