How is USB-C not common? It's the default for every remotely modern android phone I've seen, all the modern game consoles I've seen (eg, the Switch and PS5 controllers), and many other random electronics use it (I even had a covid tester that was plugged into USB-C). All my laptops these days use it (including two Chromebooks, a high end MacBook, and a Windows laptop) and of those, only the Windows laptop even had USB-A ports (ie, the other laptops only had USB-C).

I won't pretend it's perfectly ubiquitous. There's lots of older electronics still using micro or mini USB (there's been no reason for manufacturers to update older devices). But it's definitely common in my book.

Only the Pro models, I believe. Funnily enough, typical Apple too, since they are bundling only a usb2.0 cable in the box for the Pro as well.

Faster USB chipset is more expensive and potentially also physically larger with more traces on the circuit board to deal with I imagine. And faster data speeds require more attention to how the traces are routed to prevent interference. I very much doubt this is anything other than to save a relatively small amount on materials and engineering costs, on an already overpriced phone, and/or to try and "encourage" you to use iCloud by making offline sync and backup painfully slow.