It hasn't made any sense to buy phones from the company that provides your mobile phone service for years, especially now that a "phone" isn't in any meaningful sense a phone at all with many people never making or receiving any calls.

They shouldn't be any more involved in what phone you use than your ISP gets to pick whether you use a Mac or PC.

Sure if you want to use a 15 year old phone you're going to need to find somebody who allows that (my guess: no-one) just like you won't find so many ISPs who offer 1200/75 dial-up mode for your original Apple Macintosh with acoustic coupler. But anybody with normal needs shouldn't need to care, just sell me "phone" (ie mobile Internet) service.

Crunching the numbers, it was much much cheaper for me getting this phone on contract then any other route. I did look at buying a phone separately and having SIM-only or PAYGo. (UK, about 3 years ago, low-mid range). With 18 month contract I basically got a 'free' phone with 13MP camera.

I actually use a SIM only by swapping sims with my wife whose phone we bought outright (but it's low end - camera and screen are poor).

A cellular phone is also a network terminal.

A misbehaving device can cause unreasonable load on the network, a disruption to other users, or at least interruption of the service on your device.

When Apple does screw up and iOS update will leave you with no data, the operators will attempt to make it work; they won't do that for every random manufacturer though.