It depends on how you define the market. When Sirius and XM merged, did it become a monopoly on the 'satellite radio' market or did they just become a larger player in the 'radio' market?
Apple has a monopoly on the "market" (portion) that pays for apps. That's not as clean a definition of market as you would want. The defining characteristics of this market are also (arguably) inconveniently dependent on the offending practices. IE iphone user pay because it is easy, because there is a single payment provider, because users don't have to worry about a cheaper option some roundabout way, etc. etc.
Then there is the problem that any platform, regardless of overall marketshare is something like a little local monopoly. Our definitions of monopoly don't really work here but platforms can still exhibit a lot of what anti-monopoly rules are trying to prevent.
Doesn't matter? Were they abusive? Did the new enterprise fix prices or stop competitors entering the whole market (Apple are not stopping consumers from signing up for Dropbox, just not through the App Store)?
Except I didn't change the 'argument'. I asked a question. Was the new enterprise abusive? There is nothing unlawful about having a monopoly, controlling a market abusively is though.
Does a market exist where no money is being exchanged? If you are an abusive provider of things that you give away, are you a monopolist? If there are two providers of some product X where the first provider gives it away and the second provider sells it, is market share measured in units a meaningful metric of the economic activity generated by this product?