You didn't take a few things in consideration:
- Carriers have absolutely no reason to want their phones on other carriers;
- BlackBerry has, to some extent, to keep carriers happy and comply with a few agreements;
- The chipset defines which radio frequencies can be used;
Now, I am not saying I wouldn't prefer the one phone to rule them all approach. I do wonder how much more/less expensive it would be to have a single chipset that can handle all radio, maybe it would make the device more expensive? I honestly don't know. But I am sure that carriers have a major saying in this still.
On the bright side, Foxconn will be handling low-end/entry level BB10 devices. We're sure to see some dirty cheap phones soon.
