Belts come in a small variety of tooth sizes. I guess if they become mainstream, more variations will be made.
Currently, most bikes all use the same length of chainstay throughout the size range; XS to XXL, its all the same length.
You cant use a sprung tensioner with a belt, you need sliding or horizontal dropouts or eccentric bottom bracket.
The analogy with motorbikes is not correct, motorbikes use chains, belts or shafts , they all have internal gearboxes so the choice is up to the designer. Racing bikes can't use hub gears or internal gears because of the weight/efficiency so they have to use derailleurs and chains.
Belt drive frames need to have very stiff chainstays. The break in the rear triangle doesnt seem to be an issue. I dont think anyone has designed the perfect belt-drive frame yet.