You can get pretty far by using importmaps, you would not have treeshaking or a single bundled file, but it works pretty well. JSDoc can be used to add types to your project (that can be typechecked using typescript). I'm currently building a hobby project using preact, htm and jspm for packages. It's pretty nice to just start building without starting a build tool, having to wait for it to finish, make sure it's not crashed etc. But indeed, I won't use this for production.
The only thing I'm still missing is an offline JSPM/esm.sh.
You do have tree shaking: the browser only loads the modules that are imported. Only import what you use (and don't use barrel files) and you're golden.
If you don't have any external library (e.g. npm) dependency you're golden. Unfortunately, this means that you now have to write all your code from scratch, which is ok if you're writing a very light website, but it's unsustainable if you do anything non-trivial.
You mean EcmaScript Modules? The situation is quite complicated. Some libraries don't publish ESM at all (React doesn't iirc), and the ones that do often publish CJS and ESM side by side. In that case, you need to read the package.json and decide which file to use, which is not trivial (see Conditional Exports for example: https://nodejs.org/api/packages.html#conditional-exports). In almost any non-trivial case you need to write tooling to make it work, so you might as well use a bundler.
For offline esm.sh you can use service workers cache, no?
Also why not use this config in production? Http2 should give the same performance for multiple small files than a big bundle and it's much better to cache
It works well for a small website. For anything that requires more than a few dependencies, the package management is hell and load time will be insufferable. Also, not everything you grab from npm can just run in the browser even if written in ESM -- things get complicated quickly.
The only thing I'm still missing is an offline JSPM/esm.sh.