I'm also worried by this, which is precisely why I'm considering a move to Obsidian from Notion. I'm pretty much writing everything in Markdown by now, including my personal blog.
I keep losing my files with cloud-first systems. I had a trove of notes in Evernote and I don't even remember which email I used to open it, nor if they are there anymore. I'm pretty sure I wouldn't lose local-first markdown files, as I could do the same thing I do with code: keep them in git, and then have a cloud backup for good measure.