There in fact are FOSS alternatives like Joplin. Personally, I actually switched from Joplin to Obsidian due to a larger community (and therefore community-driven plugins) and overall a more polished UX. That being said, I have the security of switching back if Obsidian ever becomes evil or unusable.

Another aspect is that the entire source code is technically viewable (partially obfuscated) since it’s a web app. Having written plugins for Obsidian, you’re very much interacting with the source code itself. Feels like open source with extra steps and I wish one day they will finally make the switch to true FOSS.