I think the constant upgrading is slowly coming to an end for several reasons. First and most importantly the latest iphones cost in the area of $800 and any "upgrades" are only going to cost more. There is a limit to what people can and will pay for a phone and we are likely near it. At the current rate phones will need to be in the $1000 plus price range very soon and many people will cease to purchase the more expensive phones. People are not going to be walking around with $3k phones so there is a limit and we are likely very close to it.
Also the improvements from one release to the next are getting to be very marginal so there’s less reason to upgrade. And you can now get some android devices for a fraction of the cost that other than the status symbol do pretty much the same thing.
I don’t really know what you’re talking about. When I upgraded to an iPhone 6S I think it cost me $50. Who buys a phone outright? I have Verizon.