> Email doesn't have an easy way to add someone to a thread

You add them to the cc list.

> or to manage a forked conversation.

Your email client is at fault there, any decent email manages forked conversations.

> It's closed and non-discoverable by default.

Thats why you have mailing lists which are indexed and searchable. You may see 'closed and non discoverable' as a downside, I do not. Anything that should be discoverable should be on a wiki.