It's not about the files, I'm very happy with files being local and easily synced and messed with. It is as you say, you create a folder which Obsidian reads as a "vault" and create .md files and folders in there, plus the hidden folders that let Obsidian organise plugins...
But I'm also not exclusively using it on Android, it's my desktop driver for just about everything text. Especially please with the community plugins which make it extremely accessible for someone with additional needs when it comes to reading or writing, the recent improvements to tables and the plugins that integrate it with Pandoc and Zotero.
I was never able to replace what it was with anything except maybe Logseq, and even the Logseq couldn't replace all of the functionality and theming. I tried living a few days in Logseq, just moving my vault there, but it didn't work so well.
It's not a major issue, I would like to move to FOSS but it's not an emergency like moving away from Google is an emergency.