I like the idea, but which bluetooth device will it use to measure your distance? The mouse or keyboard? I'd rather have an option to smartly resize all font sizes, windows and panels with a key binding like you do on a browser. It would be great for universal accessibility.
The results probably wouldn't be accurate enough. Whilst Bluetooth isn't line of sight, and could in optimum cases maybe be calibrated to accurately measure, all radio waves are reflected by metal, so different positions would yield different results possibly. Also, with the GUI constantly adjusting itself, it could get annoying...
This is simply too much complexity to overcome such a minor annoyance.
There would simply be too many IFs. IF you have a compatible bluetooth device AND IF you have a TV you use for your video AND IF changing the font sizes manually annoys you, THEN this would come in handy.
And even then, I can think of many situations where you wouldn't want your font auto adjusting.
In short, there would simply be too many developer hours invested in a product that would have such a minor return.
Better they spend their time doing something else.