While you’re aboard you can only purchase food, drink and entertainment from the cruise company. I don’t think a cruise ship would ever allow competing companies to provide those services.
I’m personally a little mixed on the App Store. I don’t a have fundamental issue with the App Store model, and as an Apple customer I personally see it’s limitations as an advantage.
However I do agree that Apple using its App Store to harm competitors in other industries, like Spotify and Netflix, as very harmful. Although I don’t know how fix that without fundamentally changing the App Store, or banning Apple from competing with App Store services like Spotify and Netflix.