I certainly know what a monopoly is. Unfortunately, in this forum it's just used as a loaded term to justify certain arguments. The US courts specifically said that the App Store wasn't a monopoly in the Epic case. The EU hasn't classified the App Store as a monopoly. The DMA doesn't require a monopoly, only size metrics. You could be 10% of the market and still qualify as a gatekeeper.
So why do people keep bringing it up as if it has any legal relevance to these issues?
Agree it is at best cart before horse and otherwise not prerequisite for intervention. Being a monopoly isn't technically illegal in the US but restraint of trade regardless of monopoly status is illegal. However, as a practical matter it has been assumed that restrainst of trade is ineffective outside of monopoly situations.
Relevant text from the Sherman Antitrust Act, "Every contract, combination in the form of
trust or otherwise, or conspiracy, in restraint of trade or commerce among the several States, or with foreign nations, is hereby declared to be illegal." and "Every person who shall monopolize, or attempt to monopolize, or combine or conspire with any other person or persons, to monopolize any part of the trade or commerce..."
Note that the first part doesn't depend on the existance of a monopoly but rather outlaws any contractual attempts to restrain trade (a concept which has common law origins predating the forming of the US).
Also note the second part includes
attempts to monopolize or
conspire to monopolize any part of trade or commerce. Thus 5 competitors with 20% market share each are not individually a monopoly but it would be illegal for them to conspire to set prices (aka "pricing fixing").
The latter is not the Apple situation. As a platform for hosting mobile applications, iOS is a duopoly with Android. Duopolies can arise naturally and behave like monopolies (e.g. through tacit collusion). In general the most effective remedy for duopolies is a 3rd and maybe 4th viable competitor.
It's unclear how that will arise in the mobile OS market in the foreseeable future. At this point my ideal is a shift to webapps for most things. Then I can see why Apple has no interest in making those a first class experience on the iPhone, etc. Reminds me of how MS tried to coopt the web back in the IE days.
Finally note one area where the Epic case found against Apple was it's prohibition against developers communicating alternatives to the app store. It's unclear to me why other legal or technically enforced limits on what apps can do on the platform are not illegal but I am not a lawyer and it is not practical for me to study all the app store rules in the context of all the relevant law and precedence...