I like ES modules. ProseMirror was originally written using ES modules, but downgraded to CommonJS at some point because the compilation step slowed down my development process too much (I really dislike waiting for recompilation or accidentally refreshing before the build is done, and even with incremental builds, the build times were noticeable). It seems both Firefox and Chrome have started rolling out ES module support behind a flag. So that would make make them an attractive format during development again.
I'll do some experiments soon. Would the src/ directory (not dist/) moving back to ES modules cause anyone inconvenience? Relatedly, would having a single dist/index.js instead of different dist/* modules, with that index.js being created by rollup, be an issue for anyone?