Aren't Web Workers real threads, and supported natively in browsers? (Haven't used them myself, maybe there's some limitation that excludes them from the criteria above...)
https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers...
I also bet that I couldn't make 'in' work for proxy with empty abstract target, but maybe it's just me. Btw, 'in' and 'of' are two separate iterators, one iterates array elems and the other iterates object's keys -- something essential to metaprogramming. My whole point on in/enumerate is that it considered legacy by someone special.
And on Vue: I didn't know that, but if Vue can't take advantage of Proxy, can I?
for-in is a legacy feature; due to dynamic inheritance, it's generally not safe to use without also calling Object#hasOwnProperty() every iteration. for-of is not for "array elems", it uses the iteration protocols that are implemented for all built-in collection types, not just Array, and some DOM types, and can be implemented for any user types. Protocols are a much more flexible and clean approach to metaprogramming than overloading the for-in looping construct would be.
You can't use Proxy if you need to target legacy browsers like IE9, and Vue needs to, since it's about 15% of all browsers.
And yet even that advantage got thrown out from the language with the introduction of "import()". Apparently static analysis is a non-goal (see discussion in [1]).
[1]: https://github.com/tc39/proposal-dynamic-import/issues/35
As an aside, there exist green threads in JS, namely node-fibers, although it's only a Node.js extension.
You actually can reflect on module exports and dynamically change them with node modules; you can't do it with ES modules, but that has the significant advantage of enabling static analysis.
Proxies absolutely can trap the property assignments you mentioned, but Vue can't take advantage of this because Proxy can't be polyfilled in older browsers.
As for the enumerate handler, it would only have worked with for-in loops, which are a legacy feature. The iteration protocols used by for-of are a much more flexible solution. It might seem silly to have both for-in and for-of loops, but the context of the language is that it can't just go and break older websites. Same goes for == and ===, etc. Linters come in very handy for dealing with this.
Your criticism is better than most, which usually just point out some "wat" moments with mis-features like implicit coercion, but you didn't really make a case for having to do "all things by hand" in JS.