>That comparison doesn't really work, because MSFT had an effective desktop monopoly. By contrast, Apple is a minority player in the mobile market.
Market share is not the only criteria for determining whether a company has a dominant market position under EU competition law. Apple have a substantial share of the mobile phone market and they have a total monopoly on iOS app distribution. If you want to sell apps to ~27% of consumers in the EU, you have no choice but to go through Apple. End users might have a meaningful choice, but developers don't, which is highly relevant in this case.