This is super impressive. I would suspect that the uptake would be massive if there was the ability to import existing unity projects with little effort for the developer.
First, Unity already can do it.
Second, I am not impressed with WebGL vs what native offers, other than geek cool factor.
It also makes the GUI development faster because most of the UI is simple enough and lets the artists prototype stuff easily with tools they know how to use (CSS&HTML)
WebGL is not where the geek cool factor is, at least not yet. Native might be, isn't that why those $500 graphics cards sell? WebGL brings 3D to the non-geeks.