Is it not open source?

It locks your efforts to the vendor/project which is different to Obsidian

It is still open source, not proprietary, and you can export files to Markdown anyway.

I'm not an Obsidian user, but I agree with their "file over app" philosophy.[1]

If SiYuan stops being developed, are the files still readable/parse-able?

[1]: https://stephango.com/file-over-app

Yes, if SiYuan stops being developed, you can still get your notes exported as markdown files. Since SiYuan is open-source, you can also use the internal code to parse the JSON format notes.

It's open source, so yes, and you can export files to Markdown anyway.

Yes, your existing installations on your devices, whatever they are, can read your data as long as you use the correct Repo keys that are used to encrypt stuff. In the worst case, you can deploy the docker instance with older images to keep going on.