Wait, so the Google Play Store, which you can install alternatives to (F-Droid, Aurora, Amazon,...), and where you can easily install apps through other means (such as downloading an APK through your browser and running it from the file manager) is an illegal monopoly while the Apple App Store isn't? Well, I guess Google's market cap is only 2 trillion compared to Apple's 3 trillion, so I guess that's fair.
Look at Microsoft. They have been found guilty of anticompetitive conduct related to their open Windows platform in multiple jurisdictions, but not so with XBox.
Either never claim your platform is open, or refrain from anticompetitive behavior in the "open" market you choose to create. .