When we paired it was consciously decided that umd isn't useful to us anymore. Is satisfying what a main field expects the only reason to add it back? Is there anyway to work around that without creating a build file that we don't consume but need to maintain?

We could omit the main field entirely, and rely only on the module key.

Something to look forward to is Node's experimental support for a "type": "module" key in package.json. Once that stabilizes, we'll point main to the module file and won't need to build UMD files.

It looks like producing UMD output is fairly easy to do with command line overrides to tsc, so it would be easy to remove when we start using type: module.