> US consumers knowingly choose to buy iPhones even though they know that they're choosing a locked-in platform, so there isn't a single-brand aftermarket and there is no monopoly.

As far as I know, that has not been used as a successful defense of any large antitrust case. Ma Bell and Microsoft could have tried arguing the same point, but I'm not sure if it would override the anticompetitive damages of either company.

I linked to Epic v. Apple, where that exact argument was made and the court sided with Apple. It wasn't a DOJ antitrust suit, so maybe the rules are different there, but in that case Epic tried to make the argument that the relevant market was iOS app distribution and Apple therefore had a monopoly.

The court ruled that in order to make that case, Epic had to demonstrate (among three other tests) that "the challenged aftermarket restrictions are “not generally known” when consumers make their foremarket purchase". The court found that consumers do know about the restrictions in advance of making their initial purchase decision and therefore found that the relevant market had to include more than just iPhones.

Right, but other antitrust measures (eg. DMA/DSA) haven't been about monopoly power. Europe's measures are industry-wide changes that curtails certain first-party choices. Japan's legislation works the same way. It's entirely feasible to me that this sort of act could gain traction in the US, once the lobbyists are sufficiently exhausted.

It'll be a hard fight for sure, but Epic and Spotify go for low-blows because they know Apple will cede something in the end. Their hostile behavior in France and the Netherlands did not go unnoticed.

Your take on the consumer perspective ignores network effects. Lots of people, particularly children, buy iPhones just to avoid the dreaded green bubbles. Adults may be better at handling this, but it's hard to tell children to just use Android if that means they'll be bullied. I don't think it was originally Apple's intention to conscript legions of schoolyard bullies to increase their market share, but it's certainly happening, and they certainly don't mind that it is. Interoperability (particular opening up iMessage and iCloud to other devices, the same way anyone can use RCS or Google Photos from an iPhone) would mitigate that.

Your take on the developer perspective is a bit confusing. Everyone knows it's a closed system, but that doesn't mean it's not a monopoly. In many ways, it is a monopoly.