The supermarket on the corner also has a monopoly on what gets on their shelves, and which persons are allowed to get in to buy their stuff.
I can choose from 4 different supermarkets within walking distance, and an additional 3 if you count online supermarkets that deliver to my area. That is not the case when it comes to choosing a mobile OS/app store.
Having more than one to choose from is still an option.
And I also know a few that still use and buy feature phones with their OEM only apps, with "by invitation only SDKs".
Which makes the choice bigger than two actually, when one looks at the actual market distribution, something that a judge will care about.