It is an ESM problem, since it works just fine with non-ESM script tags. ESM isn't some theoretical thing, it's the reality of how it works, in particular when we are specifically discussing the practical reality of using it. And the fact is that it absolutely is more cumbersome than the old technologies (and more cumbersome than alternative approaches that could have been taken). The fact that we're now talking about launching Chrome with special command line flags demonstrates this. I mean, the rest of my comment that delves into all the other problems proves it even more, but the fact that it fails the bare minimum requirement of just loading when you drag and drop a file into the browser is fairly bad.
> It is an ESM problem, since it works just fine with non-ESM script tags.
This is not my experience. I've had many cases where Chrome security blocked non-ESM script tags to file: URLs (that Firefox was much more fine with).
The security mechanisms in the browser have gotten extremely sensitive (and rather baroque) and blocking file: by default is a part of that. This isn't an ESM-specific thing in any way, this is an overall browser security thing. It is a response to the arms race of malware.
People that have been writing/developing apps and scripts designed for local use (such as Twine) have been complaining about Chrome security defaults for years now for non-ESM development. "Drag-and-drop" of Twine games is generally only reliable on Firefox. There are so many obscure Chrome security issues that can break such games. This is not a new complaint for ESM.
(I didn't break down the "problems" with the rest of your comment because it was a bit of a rant and a lot of it felt to me more like opinions and bike shedding rather than "problems". I didn't think I can convince you that ESMs are good and wasn't going to try. I can point out the facts that script loading from local files is a Chrome [and IE] problem more than an ESM problem, because the facts back that up.)
If you expect to be able to develop with the file protocol you're going to be disappointed. Essentially all modern features don't work with it; not just ESM. You can't use Service Workers with the file protocol either.
