But no substantial modern application can possibly be built without them.
I can see that using modules could avoid the need for compilation.
That's certainly false, since substantial applications predate React, Angular, svelte, etc.
But you have a point. Steve Yegge once wrote that his company wanted to write a word processor in assembly and it seemed like a good idea at first, but then it was like drawing a straight line 100m long using only 2mm line-segments. The major purpose of these libraries is to increase the "length of the line-segments" to something more consistent with the distance you want to cover. The question is whether or not their exists a unit of abstraction in raw js/html that serves this same purpose. E.g. it's a demonstration proof.
Working on it.
Check out Firefox.
Preact has a no build path in their documentation: https://preactjs.com/guide/v10/getting-started/#no-build-too...
And here’s my no build react setup: https://github.com/jsebrech/create-react-app-zero
Of course, unless you do it tailored for your project, you don't get some nice features like tree shaking, but it's very convenient.
But what is this "build-less front-end" you speak of? Surely there's build somewhere, you're just hiding it?