I read everything you said in full and in my own opinion the DoJ is doing the right thing here. I do feel that their market share in the US is artificially held high by lock-ins and abuse of their market position.
And yes I do see the 88% teenage ownership metric as a concerning trend because we know from all markets (whether it's shoes, vehicles or phones) that when you get someone hooked on a particular brand young they are more likely to continue buying that brand their entire lives. So 88% of teens today could translate to 86% of adults in another 10 years from now.
And that isn't a problem in itself if Apple is playing fair and the friction to switch platforms isn't high, I just don't personally believe they are playing fair. My personal belief based on my own interpretation of the facts is that they've abused their market position and I largely agree with the DoJ's assertions in their legal brief as I have also done with the EU's legal cases against Apple.
Now I'm not saying the DoJ is going to win every part of their case. There are parts I disagree with that I outlined in my initial post (Amazon's phone failure for example) but there are I feel some strong cases to be made for Apple not allowing sideloading, not allowing third-party app stores on the iPhone, not allowing 3rd party browser rendering engines, not allowing full integration for other messaging platforms, not supporting RCS, not allowing other smart watches to integrate properly with the iPhone by denying apps that would attempt to bridge that functionality gap etc
I think I fully understand your position here, I do comprehend what you're saying. I just have a difference of opinion about the same facts that we both agree to be true.