Glad you liked my post!
If your iPhone is the only device you own that uses Lightning and you trade it in for an iPhone with USB-C, I don’t see why anyone would keep their Lightning cables around unless they buy a dongle for it, which would be more expensive than just getting new cables. I guess you could keep the cords for AirPods and older iPads, but they aren’t used for much else.
Your point about USB-C being a “pro” feature is a good one. However, I’d really be surprised if people were plugging their iPhones into external monitors or TVs, which you can’t do with Lightning. That makes more sense with the iPad, which is probably why Apple hasn't been hesitant about putting USB-C on the iPad Pro.
I think USB-C on the iPhone would make sense in terms of transferring large video files, especially since iPhones can now handle 4K Dolby Vision, but wouldn’t iCloud transfer that to the Photos app via Photostream even if you aren’t buying extra iCloud storage? In general, I don’t think Apple sees (or enables?) the iPhone to act as a hard drive, but I think you’re definitely right that it would fit the definition of a “pro” feature.