While this sounds like a good idea.
In practice it is not. You are increasing weight, thickness, and lag between screen and device.

On top of that, the screens can't be generic.
They would have to be specially tailored. And this means more time, more money, less units.

If you're interested in something like that, see Google's Project Ara, the modular phone.
I believe cohesive devices and services will (always) win out.
In fact, its done so in may fields... not just in smartphones, eg:
- disposable razors are more common
- cars are built to be disposable and recyclable
- recyclable packaging on many products
- even health corporations want to treat symptoms instead of find cures
- etc etc

It's not the early 1900's where things were made to last : (