I would argue the benefit is just making everything much simpler. You never need to think about module systems and interoperability -- there is only one module system to deal with.
That proposal seems to have stalled, and I doubt it's going anywhere. Here is the thing: people want hard, strict compiler checks that can be enforced at all time, not Python-style type hints. If you want JavaScript but type checked, Typescript is the de-facto standard. Either do "checkJS" or convert the project to Typescript. In fact, the latter is much better in terms of expressiveness.
It only makes it simpler if you have a new project and can be ESM pure. If you are working with existing code or need non-ESM modules, everything is much more complicated.
That proposal seems to have stalled, and I doubt it's going anywhere. Here is the thing: people want hard, strict compiler checks that can be enforced at all time, not Python-style type hints. If you want JavaScript but type checked, Typescript is the de-facto standard. Either do "checkJS" or convert the project to Typescript. In fact, the latter is much better in terms of expressiveness.