Not sure why that would be, surely while Apple dictates their asking price they don't dictate financed price do they? So that's a self-inflicted wound, and one which carriers could fix by financing the phone less.
On the other hand, the iPhone precludes almost all carrier control, they don't control the initial load, they don't control the updates, ... Carriers control some features Apple gives them hooks for (e.g. tethering), but that's about it. And customers become loyal to the device (and its manufacturer) rather than the carrier, not hesitating jumping ships to a carrier providing the device.
> Carriers need a third player, to push down financed device costs
It's not like Apple had much say in this back in 2007~2008, and BlackBerry was still there...
I don't see sensible support for this narrative.