Oh, I totally don't blame you for not wanting to implement ENEX export. I can't wait to escape Evernote's clutches. I do remember when Apple first launched iWork, though, that they made "round-tripping" MS Office files a priority, not because they liked the format but rather as a way to make people more comfortable switching.
I think the broader issue for me, though, is maintaining some semblance of metadata portability, which Markdown, HTML, PDF, and plaintext don't support particularly well. (I also strongly distrust file system metadata.) My main use case for Evernote is actually just treating notes as freeform metadata for attachments, though I do have plenty of actual notes. Evernote is incredibly mediocre for this purpose, of course.
I actually did succeed in importing my 17,489 notes into Joplin, only to find that they had all gotten dumped into a single notebook (albeit with tags intact). So it seems like I will have to do more "preflight" for my Evernote before I'll be able to finish switching. (I think the easiest thing would be to duplicate the notebook hierarchy as tags, rather than exporting a hundred or so different notebooks individually.)