Market share is relevant if they're going to be prosecuted for the same thing Microsoft was prosecuted for: abusing their monopoly power.
Anchor v Rule:
although market share does not alone determine monopoly power, market share is perhaps the most important factor to consider in determining the presence or absence of monopoly power
Weiss v York Hospital:
A primary criterion used to assess the existence of monopoly power is the defendant's market share.
Maybe Apple can be prosecuted under some other legal theory, but that's a separate question.. and if we have to get into that, that's just more evidence of my point that Apple and Microsoft are different situations and aren't comparable.
You can't really compare the US to the EU though and this isn't going to the US supreme court.
You don't need to be successful in stifling competition to be anticompetitive so market share enough to be a "real" monopoly is likely irrelevant. Trying to abuse your position is enough as Google felt with Google shopping and Android. "Capable of having, or likely to have, anticompetitive effects". In Android, "it is sufficient that the conduct tends to restrict competition or is capable of having that effect".
In my opinion Apple is going to be hit hard in the EU. Fans of Apple always roll out the old "but the app store is much more profitable" which is a truth that will IMO come back to bite Apple when refusing others fair access and thereby "restrict competition".
>that's just more evidence of my point that Apple and Microsoft are different situations and aren't comparable.
I agree they are very different and not comparable.
Anchor v Rule:
although market share does not alone determine monopoly power, market share is perhaps the most important factor to consider in determining the presence or absence of monopoly power
Weiss v York Hospital:
A primary criterion used to assess the existence of monopoly power is the defendant's market share.
Maybe Apple can be prosecuted under some other legal theory, but that's a separate question.. and if we have to get into that, that's just more evidence of my point that Apple and Microsoft are different situations and aren't comparable.