Currently the code under server uses ESM and the remaining code is transpiled to CJS. Ideally we would be able to set "type": "module" and drop the .mjs extension for the server code. Of course, the React code under pages and component would still need to be transpiled.

The last time this was attempted, there was some issues with Next.js:

https://github.com/vercel/next.js/issues/9607
https://github.com/vercel/next.js/issues/24334
https://github.com/vercel/next.js/discussions/22381