It seems they would like the flexibility to administrate the server any way which suits them. Using someone else’s server would complicate that, and expose their discussions to unknown administrators.

Those are just the things I can think of right off the bat.

You’ll like it. It’s really versatile and has been around for like 20+ years so lots of support for it. It can be pretty comprehensive.

If I understand what I’ve read, you may be interested in a SIP software called asterisk. It may do what you’re looking for.

Via their Sync service, yes. If you’re in the Apple ecosystem, they officially endorse iCloud to sync your files, which I personally use. They discourage using Dropbox, but I reckon it’s possible.

The thing I most appreciate about Obsidian is, for now at least, they at least partially embrace a sort of FOSS mindset in that they offer a proprietary thing via a sort of compromise: your data is stored in plain text in markdown, so it remains 100% portable and parseable by anything which can parse markdown.

But I get what you mean.

So many—almost too many—extensions!

Obsidian.md hands down if you can transition to markdown instead of rich text. Lets users have wiki style hyperlinks to notes.