Browsers will catch up with what developers want.
We have seen that with ActiveX, Flash, Java applets, jQuery etc. Every time tech was needed because "The browser alone is not good enough" the browser became better and made the additional tech obsolete.
As more stuff gets done in Javascript, and Javascript gains more APIs, browsers like Lynx, Dillo, Netsurf, and umpteen yet-to-be-written ones are becoming less able to browse the Web. Likewise, the effort required to "upgrade" them would be massive.
I think a consolidation of features would alleviate the situation a little: have a small "foundation" of required tech, e.g. a subset of JS (or WebAssembly) with few primitives or APIs, and have the browser pull in polyfills for everything else, including interpreters for fuller, newer JS standards, etc. If a browser is graphical, it could provide a bitmap-blitting primitive, and use polyfills for CSS, fonts, layout, etc.
The Chromes and Firefoxes of the world could ignore the polyfills and keep going with their highly-tuned native implementations of these things, but it would prevent "kicking the ladder away" from everyone else.
JavaScript is not their biggest problem, as they can plug in one of the existing engines (in fact one of the links forks does just that).
As someone who attempted this for Lynx back in the [~late 90s~] early 00s with the Mozilla JS engine of the time, no, they can't.
Lynx has no DOM. It parses and renders the HTML to a fixed textual pane without any intermediate format - even `document.write()` is nigh-on impossible to implement because it required keeping a copy of the HTML around, jiggering it with the output, reparsing, and going through the layout again. And that only covers the most trivial uses of `document.write()`.
To add Javascript to Lynx would require rewriting the entire internals of Lynx - not going to happen.
[Edited to correct the time period]
they are still able to browse the web that they were able to browse back then. You can't expect that the web won't move on.
The only lament is that most websites don't cater for text only/low-resource/disabled users. But that's a different story to the OP of having lots capability/libs/standards.
And almost every time, a separate browser codebase died and a vendor switched to WebKit. Because with all this cruft, maintaining a browser codebase has become a nightmare only few can afford.
Perhaps this is not all too inconvenient for those few ...
Developers want everything from JavaScript to C++, Python and Haskell. There's no way a browser can specifically address all what developers want. Therefore browsers should become better virtual machines.
Edit: yup, WASM is a good start.
That's what many think, but that's not what WASM is for though. It's still a foreign world to the DOM, and not a first class citizen like JS.
It's more about driving canvas games or offloading some expensive computations than actually writing your app in WASM as opposed to JS.