Is that better? Become successful, get sued? Android has 87% market share, iOS has 13%. That's not even a monopoly.
Of course we can define arbitrary categories like "Apple has monopoly on the iOS market". Which makes the concept of monopoly absolutely nonsensical, because then everyone has a monopoly. I have a monopoly on the slver username on HN, so I guess EU might sue me any moment now.
Also, let's recall EU suing Microsoft and forcing them to offer Windows without a media player. So what did this result in? It resulted in lots of nephews children and grandchildren having to visit their relatives and help them install Windows Media Player. I'm from EU and I want to like all their decisions, I'm team EU. But they're complete idiots sometimes when dealing with tech.
Yes it's a duopoly and yes they are both abusing their market power. It got so bad that you have to get testimonials of mobile developers anonymously against those two companies because they are fearing retaliation against them (yes that does sound like a mafia).
Not only that, this change highlighted the App Store and Play Store cartel[1] that engages in price fixing[2]. Google also dropped their prices to match Apple's, but not any more or less.
Instead of mobile app distribution prices being dictated by the free market, they're dictated by the cartel. Prices have only changed once in a decade, and not in response to the market at all, but by the whims of the app store cartel.
Because they never had the market share and power in the music market that they have now on the mobile market that's why. You kind of answered why they are getting sued now yourself.