Your set up works because you tailored both the hardware and the software to make up for the shortcomings of the devices. Normally a TV remote doesn't have a full fledged keyboard attached to it, so many applications become borderline unusable. What you did is enhance the inputs of your device and this opened up many more possibilities. You also tweaked the software from its ootb configuration to make operations smoother.

None of these devices attempt doing something similar. They offer less options than normal computers and their desktop is left unchanged.

Lastly, what you wrote actually proves my point. You are not using your linux system to do everything a computer does. Your set of functionalities is limited to media consumption and as such you just had to find a workaround for your use case. Could you browse (or even write) your emails on your TV? Sure, but you'd have to use a heavily customized UI and UX if you don't want people to squint their eyes and take ages to navigate everything. And who's going to write such a program? And what about all the other applications that we expect a complete system to have such as a general purpose web browser, a video player, a file browser, etc.? My whole point is that these "pocketable PCs" might be great for a subset of problems, but they have to be tailored for them. They are terrible general computing devices.