Obsidian doesn't just render markdown though. There's a ton of functionality on top of Markdown which makes switching to any other tool very hard in reality. This is further exacerbated once you start relying on plugins (which arguably is the case with the majority of Obsidian users).

I mean, we're not talking about a hosted service here. Albeit not OSS, the client is free, API stable, fully functional offline, and very extensible. Even if Obsidian the company went away, the latest version of the app would continue to work and you would still own your data.