Well the phone is already a trackpad, you can mirror the screen wireless via an Apple TV, and you can use a Bluetooth keyboard. In fact, I’m doing all 3 to write this (The user-experience is more than acceptable, but I’m not going to try to use the setup for coding any time soon).

Apple could pull the trigger on running full OSX on a phone or iPad if they thought there was a market for it but it would be a backwards-looking step.

It’s not exactly an innovation leap to think “hey, phones will match desktop PC performance, we could plug one into peripherals and make a device that can become a desktop”, everyone has had that idea. It just seems a bit backwards looking? In 10 years time the power of a current Mac Pro will probably be available in a watch with 5G+ bandwidth. If necessary, a desktop OS experience could be an app or a cloud service on whichever screen (or headset) is available.

TDP matters for serious work. Over normal work periods, that ipad is going to continue to operate at a fairly high frequency while the iphone will throttle because it can't dissipate as much heat. Dissipation and size affect many other parts of system design too (eg, memory speed, width, and size).