The US Supreme Court disagrees with you.
Apple monopolizes the distribution of applications on their stores, hurting consumers. This is a supreme court decision, not just about this thread, but Apple does absolutely use its app store position to harm consumers.
https://www.supremecourt.gov/opinions/18pdf/17-204_bq7d.pdf
Plus your strawman argument is inappropriate. When you own your car, you can do whatever you want with it. 3rd party vendors can sell upgrades or modifications to it (at your own risk) and BMW doesn't have to give 30% of their sales to BMW.
There is no BMW market. Or if there is -- I'm not a car-mods guy, but -- it's entirely adhoc and not directly tied to a vendor. Like I said, you can go to Car Toys and have your BMW turned into a Ford with a Tesla screen if you want. But BMW gets no cut. That's allowing you to do what you wish. That's an open market.
Now if BMW sold custom seats for their cars, they could also do whatever they wanted.
But if BMW allowed 3rd party vendors to sell seats, but only through BMW, and somehow was impossible to change seats or forced 3rd party vendors to give 30% of their money to BMW, now we'd be getting close to an appropriate analogy.
By the same token, you can jailbreak your iPhone, you can throw it against the wall, you can paint it rainbow colors, you can run Windows on it. Do you see the difference here? Monopoly != Monopolistic behavior.
Apple absolutely has total control (not saying monopoly) of their own individual market, and engages in harmful monopolistic practices using that market control that hurt developers and consumers. That's why the supreme court decision stands, and now antitrust lawsuits are in play. Apple makes a play about how many billions their developers make. And Apple does very very little to protect both consumer and developer here, it's been basically noted that this cut is arbitrarily for infrastructure that doesn't need anywhere close to that upkeep.