With something like Svelte, I see no reason an editor couldn't let me choose if I want to see the JS/HTML/CSS in the single file (as it is on disk) or split it up into separate editor tabs for me.
What? How is that a problem, why would it be something that's best avoided, and why simply displaying all three files at once in one of a million ways available is not a solution?
I don’t see any added value of separating pieces of a component into different files. The CSS, JS, and HTML are all logically coupled no matter how you organize them.
At best you will achieved a few smaller files. At worst you make working within the system a real pain.