While you are correct on every point, it's worth considering that Apple will most likely struggle profoundly with selling any Lightning products post iPhones 15 launch as Apple can't bet on two horses at the same time while seeking to transition entirely to USB-C:
iPhones are Apple's best selling devices and the big news story that nearly all consumers, techies or not, will hear about and grasp the headlines from.
Thus, Apple can't successfully position Lightning as still being valuable and a contemporary spec while also advertising iPhones 15 as the latest and greatest with USB-C and its many advantages as reasons to upgrade, with faster charging and higher data-transfer, etc.
All things are pointing towards Apple going full USB-C for every new product post September 12.
So while they theoretically could sell Lightning equipped devices for many years down the line, I think switching as many devices to USB-C as fast as possible will avoid Apple and retailers having to give steep discounts on Lightning devices post iPhones 15.
The smartphone market is already slowly stagnating more each year at large.
Apple has to provide steep discounts for SE, 13 and 14 or refresh the stock with USB-C versions of them. I think, at the very least, Apple will do it for iPhones 14 and 14 Plus considering how close their specs will be to their upcoming 15 and 15 Plus counterparts.
I doubt the cost of refreshing older models to feature USB-C would be worth it to Apple. They’ll push folks to buy the iPhone 15 if customers want USB-C.
Otherwise, I don’t think Lightning vs USB-C matters much for older models. I’m betting Apple can get away with their usual $100 price drops. We’ll have SE3, 13, 14 on Lightning and 15 on USB-C this year. Give it a couple more years and it’ll be all USB-C (likely SE4/5, 15, 16 and 17).