There is - you can just install NoScript to block all scripts and stop video autoplay in Firefox setting. Presto - a slim web. (Sites that need JS often have a slim alternative - there are superfast "lite" or "mobile" sites for GMail, Reddit, Facebook and many more.)
Is there something less aggressive? Firefox already stops auto play based on a permission. If you explicitly click on a video then it allows you to start the video within the mouse event.
There are lots of reasonable use cases for JavaScript. Think of something purely event driven like sorting a table by clicking on columns or an input field with autocomplete where you see a preview of results as you type.
I think the web in and of itself isn't the problem. I haven't checked but I don't think any JS framework asks for gigabytes of memory. The problem is browsers prioritizing speed over everything else.