As counterexamples: Apple sell Logic, yet it has numerous competitors, also all fairly successful: ProTools, Live, Cubase, Reaper, Ardour, FruityLoops. Apple give their customers Notes, Reminders and Mail for free, on all their devices (i.e. you don't even need to get hold of apps for these functions), and yet we also have Evernote, Notion, Airmail, Spark etc etc.
Does the App Store monopoly significantly change the nature of app competition? I'm not convinced, but I'm open to learning about it.