The way it works in general is that apps support importing various formats, and then export their own format, plus open formats (for example Joplin can export to Markdown, HTML, PDF or text files).
It's rare that an app would support exporting to a closed, proprietary format, unless perhaps it's extremely popular (PSD or PDF for instance makes sense). So that's why I'm not keen on supporting ENEX export, or else why stop there? There's a ton of note-taking apps which are equally or more popular than Evernote and maintaining all these export formats is not feasible. Again, this is also the reason why it's the app's job to support various import formats, rather than expect other apps to do it for them.
Have you considered asking on Evernote forums to support importing the JEX format? I know how this sounds, but could be worth a try and that would be more logical than us supporting ENEX export.