Over two hundred phones have been ported to postmarketOS and every person giving it a shot will improve it.
It's not that cut and dried.
A look at the postmarketOS devices page reveals:
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"the most supported devices, maintained by at least 2 people and have the functions you expect from the device running its normal OS, such as calling on a phone, working audio, and a functional UI" (aka what you need a phone to be); Device count: 5, not a single one of them the kind I can go into a regular phone shop and buy
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"Devices that have had a lot of work put into them, where regressions are actively fixed, and the port is overall in a pretty good shape (read: your experience will likely be bumpy and not overly smooth); Device count: 28, largely older devices (pre-2018, so again not something I can just go and buy, and exotics like above; There is a lot of orange in the features table)
The rest is under "Testing", and the best summary of that status I can find is "All the devices in this table can at least boot postmarketOS. To monitor boot progress, you must be able to receive output from the screen, a network adapter, or a serial port". So there is a total of 33 devices right now, largely exotics and older devices, that you could reasonably use with postmarketOS for any purpose other than testing and tinkering.
I am what you'd call 'tech interested'. I tinker with Arduinos and solder electronics as part of my hobby. I do a smidgen of self-hosting and similar, though I am not nearly as far into the weeds as many people, and it's not my key interest or activity. The thing about a phone is, I need it to work, because I need it for work. I don't have time or compunction to go through the process of installing an OS the manufacturer doesn't want me to install. I don't have time to deal with a non-polished UX or capricious apps that need workarounds to install on a 'non-standard' OS (for lack of a better term). I know that's not the fault of the OS, but a choice made by phone manufacturers and app developers, but that doesn't make it any less real or an issue for me.
The USB transfer speed claim is misleading to say the least. The iPhone 15 was already capable of up to 10Gbps transfer speed (USB 3.0 support). You could quibble over the fact that the included cable didn't support that (if only the USB-IF could get its shit together), but to claim the hardware doesn't support it is a lie.
Also, non-US iPhones support both physical SIM and eSIM.