Imo, the real problem is that most people believe what you're saying is true, but if we assume that JS itself is not the problem, and that finding the right patterns for building fast websites with JavaScript, the next question becomes: who do we look to to learn the right patterns from.

For most people, the intuitive thing is to look to successful companies: Google, Facebook, etc. The problem here is two fold: (1) operating at scale often has requirements that supercede making individual pages fast for individual visitors and (2) massive market dominance/semi-monopolies/vendor-lock-ins means these companies don't really need to compete on perf. See Facebook.com/gmail.com as lovely examples of some of the worst performance on the web.

You're right that one can make a webpage fast with correctly implemented js, but I disagree that people can make webpages fast en masse with correctly implemented js; dissuading the use of JS is much more likely to have a positive effect than expecting people to somehow spontaneously learn how to write good js.