Hm – Client-side web programming is all about resource management. I can't see, how cross-compiled C++ code that originated for a completely other platform with object code in the MBs range will do a decent job. And even, if this is not the original idea of WebAssembly, this is sure to come.

(We've seen similar with emscripten: As much as I love it as an experiment, it's highly problematic in production.)

And of course, as others have already pointed out, it's likely to ruin the vision of the web as a mosaic of just a few trivial technologies that can be investigated with modest knowledge. (Compare the comment by zmmmmm.)