"Proprietary" as in "sole proprietor" is appropriate for a project with zero governance, launched by Google after some incubation closed-source, dominated by Googlers.
NaCl is not adopted because it's machine-dependent!
PNaCl is not ready. Show me Chrome Web Store games compiled with it and not NaCl, then we'll talk.
As I've written before on HN (see https://news.ycombinator.com/item?id=2998374, "I've paid my dues"), JS was created by me in a tearing hurry in 1995 for Netscape, the would-be market power that nevertheless avoided a monopoly conviction (unlike the other guys).
There is no "quasi-monopoly" here. Someone on HN schooled me on "monopoly" (http://news.ycombinator.com/item?id=2998590).
The issue with JS is not "monopoly" in the econ 101 sense. The issue is that JS is more than good enough, and getting better under competition and cooperation in the standards bodies. Therefore it is very hard to displace, and just as hard (if not moreso: a displacing language might be backward compatible) to supplement with a second language/VM in all browsers.
You should respond to this technical fact (by which I mean, the circumstance is well-founded in software costs).