If iPhones were cars, you could buy apps from anyone without going through Apple, just as you can buy consumables like gasoline and oil, repair parts, accessories, tires, etc. without going through Ford.
There is quite the legal / judicial precedence for control of (video) electronic platforms. Atari vs Activision (back in the 1980's) established the legality of 3rd party developers - in the first place. And secondly, that anyone (i.e. Activision) could write software for the Atari VCS since there wasn't anything actually blocking it. That is if you 1) Either could figure out how it worked on your own or 2) Had previous knowledge of how to write software for it (e.g. were former Atari employees) - There was no action or legal recourse available for Atari to prevent anyone writing software for it. Henceforth, this is very specifically -*
why*- every electronic platform since then has incorporated some form of lockout feature so that 3rd party development could be controlled. This lockout feature is the copyrightable portion of the system that has the legal protection from circumvention. 3rd party developers (e.g. Tengen) have tried to circumvent the lockout method in various ways but every successive and successful circumvention has been found illegal. Nintendo was sued and found guilty of being anti-competitive in their licensing requirements - but not that they couldn't set requirements (fees, restrictions, quality levels, etc).
As I understand this does not just apply to video game hardware. This is why cellphone and tablet hardware manufacturers are allowed to develop a method to "lock out" anyone from writing their own OS for said device - if manufacturer so desires. It seems to me like you might be ignorant of this history.
Neither Sony or Microsoft or Nintendo are required to allow "3rd party" e-shop game stores or "side loading" of independently developed video games for their respective video game hardware platforms. What makes Apple's "App Store" any different from PSN, Xbox Live, or Nintendo's e-shop.
Moreover, as Apple creates non-cross-platform features that users grow dependent upon (e.g. FaceTime), it creates barriers to changing platforms that at some point will be so severe that they will effectively bifurcate the market for mobile software just as surely as a chain of mountains bifurcates a physical market. So arguments that "iOS apps are not a market" are misleading. What they are really claiming is that iOS apps are not an independent market yet.
I'm not sure why you chose FaceTime of all possible things as your "severe" lock-in example. FaceTime is terribly behind all other video chat software and services. Moreover, iTunes music purchases are DRM free - purchased songs can be transferred to any device and any system and even transcoded to MP3 (or any other audio format) should said device or system not be capable of playing AAC audio. Specifically, you have Apple of all businesses to thank for effectively killing off DRM on download music sales. iTunes Music, the streaming service, isn't Apple exclusive either. iTunes is available for Windows PC and and iTunes Music app is available for Android phones.
What I could possibly see ending up being costly or a burden down the road for someone looking to switch from iOS to Android would be the loss of (or repurchase cost on Android) of iPhone or iPad apps that have Android counter parts. But, App replacement cost is going to be bi-directional anyone moving Android to iOS is going to face that cost. So this leaves just movie purchases / movie redemptions. Which is going to be a hit or miss - as not everyone redeems dvd/blu-ray disk companion iTunes movie download codes or purchases movies directly from iTunes. Because there are so many competing ways to rent, stream, purchase, or watch movies besides iTunes. And push come to shove for someone who really wants to port their movies - the FairPlay DRM is crackable and losslessly removable.
I want to give you the benefit of the doubt here - But, I don't see it. FaceTime, iMessages, Siri, iCloud, iTunes, Continuity or Handoff, even AirPods - these are all nice things, but they are parlor tricks. I'm calling you crazy if you're saying that losing these would be "so severe" as too keep anyone anchored to iOS. What future are you seeing - and why isn't Androids far less restrictive and more open platform more desirable to both consumers and developers in this future?
The only reason Apple can get away with such restrictive policies across their entire platform, of course, is that developers have no choice in the matter, because there are no third-party app stores on iOS
Again this is entirely because of Atari v Activision from the 1980's. While 3rd party software development is inherently legal - hardware manufacturers are unquestionably allowed to manage 3rd parties through lockout methods. No, they can't be completely arbitrary about it or anti-competitive, but they absolutely get to lockout 3rd parties who don't agree to their terms. Google chooses to allow side loading and 3rd party App stores - but
they don't have to. To that extent they can totally change their minds, too.
Google just banned Chrome extensions being installed from 3rd Party sites.
the alternative is to cede half of all U.S. cell phone users to their competitors.
Uh... Android consistently holds around 80% of the total cellphone marketshare. Every Android fan will remind you of that. The
smartphone market and premium smartphone market are just fractions off of the total cellphone market. I don't buy for a second that "half of all U.S. cell phone users" are iPhone or Android. I'd think Apple would want to put that into a pie chart just like they do when they announce iOS version adoption percentages. Rather, I'd say it's pretty damning if Apple has half of the US (let alone increasing that market share). I mea, Android is touted as having so many advantages particularly in the "most important" areas: such as user replaceable batteries, SD card support, UI customizations, a more advanced UI, and 3rd party app stores and app side loading - not least of which is not skimping on or being behind-the-times on hardware. If consumers are disregarding all those advantages for the restricted and dated hardware of iOS devices - I might suggest that these features aren't as important overall as android fans continually claim they are.