There's significant danger in the fact that people don't know that they need to learn it. They just write what they know, and techniques that are actually supported across all browsers they're supporting (with IE8 or hopefully IE10 being the minimum supported browser) lie unused for quite a few years. That these techniques need deep knowledge to make a solid result indicates a point of concern in the suitability of JavaScript for such things. Are ES5 and ES6 merely papering over the problems where a fresh-room approach might work better? (My answer: I frankly don't know. In the very long term, replacing JavaScript might be a good thing, but it would wreak significant havoc for quite a few years and would be impressively difficult to orchestrate from the browser manufacturers' perspective.)

I'd still love to have the safety of a language like Rust for modern web development. At some point, someone will fix Emscripten support up for it, so then it'll be comparatively alright. (I like the theory of the one-language-from-end-to-end approach, but I want it to be a safer language than JavaScript.)