If you have package.json file you can still use .js file with 'type: module' in package.json.
I’m thinking about new projects rather than converting existing projects.
One benefit I can see to .mjs is that if all extensions are .mjs it’s clear what type of project it is without the need to open up package.json.
About new projects, I'd have to read more about this new extension, for some reason it feels like a temporary solution that'll just get merged to the normal js extension in a future release.
From what I’ve read though, there doesn’t appear to be much of a plan apart from the current solution.
My initial reaction is that I want to keep all my files with .js extension.
One thing that could potentially be problematic is for files that need to run in the browser and on the server.
Really curious to see where the community’s tea leaves settle. My guess is that people will eventually just start using the new extension.