"Starting today, three billion devices can render WebGL content"
umm, there are a staggering number of iphones, not all of which can upgrade to ios 8, & their number does not total 3 billion. The release of ios 8 will add a lake to the ocean of webgl capable devices, i don't think it'll suddenly kick start the adoption of 3D content on the web.
I would suggest the lack of demand is because no one really wants 3D. How many 3D channels are ther on tv despite loads of people having 3D capable tv's?