I'm sure Joplin is excellent and I hate to argue one against the other. The markdown files in themselves should be independent. I have used several full featured, rather excellent, note-taking software, where the export is not "open". Obsidian is the only reasonable system I have seen where the software is up to date and it doesn't do anything with the markdown files to lock them in. I speak of this as a single user experience. And no, I don't like the idea of Mac Notes and Windows OneNote either. I'm all for note-taking apps which keeps the data/files independent.
I recently migrated from Evernote to Joplin. This was a concern for me, as well. However, I got past it. This isn't a case of "lock in". Even if Joplin loses mindshare and stops being actively maintained, I know that I'll always have a version available (even if only locally on my desktop, seeing as how I install it manually) that I can use to export to plain, raw Markdown files. And, with them, I can migrate to Obsidian or whatever comes into vogue.