Wow!!! I didn't know about web assembly!!!
This said, We're not there yet, there will be probably a lot of opposition, like there was with asm.js (people saying we should rather optimize JS engines, as there is apparently lots lots of room for improvement).
Also, writing browser code fully in C++ would be a huge step backwards in terms of usability for software developers. Probably the thing that is going to happen is rather that JS will stay the go to language, and small CPU-intensive functions will be reimplemented using webassembly.
And anyways, even if al of this happen, that won't be for another 5 years before it lands ... and in 5 years so many things can happen in software world (skynet etc ...)!