I think you misunderstand what apples value proposition is, at least nowadays. The app store and not being able to use other app stores is not a reason people get iPhones. Maybe back when app stores were first created and the threat of malware was greater people might have considered it but nowadays no one cares. Even the idea of a unified ecosystem isn't as much a selling point any more because Google and Samsung offer similar seamless integrations with their accessories. You can see this in their marketing, they aren't focused on how all the apple products work together easily any more. In their marketing you can see what they think their value proposition is, and what was their big Superbowl ad this year, longer battery life ...

Apple at this point knows it doesn't have much of a value proposition for switching from android. So the only way they're gonna sell new phones is to get the kids who don't have a phone and convince the people who do have an iPhone to get a new one.

They convince the kids through their tried and true aesthetics and lifestyle marketing, this is about half there marketing these days. This along with iMessage in the U.S. and the general fear of being in the out group and obsession with brands that younger people have moved them towards iPhones.

They convince the current users with incremental upgrades, eg. Better battery life, better camera; and maintaining the walled garden and keeping exit costs high so they don't turn to androids for those incremental updates.

All this is to say that apple having a single app store isn't a sign of consumer sentiment, but a sign of apples desire to milk as much profits out of their current users as they can. Other app stores can only benefit the consumers, either they do get them for lower fees or don't because they put some value on the "ecosystem". From a company's perspective yes your right that they want to be able to do anything to their product they want, but the goal of regulation is to step in when the companies desires are at odds with the people or the consumers desire, this is one of those cases.