Just to clarify, most users of Zulip were using the central cloud service (zulip.com). Kevin's experience is from one of the Zulip customers who had "Zulip Enterprise" (aka a Zulip server in their own data center -- which is also the basis of the "self-hosted production server" installation process you see now is based on). So "turning it off" in this case refers to their discussions about their internal deployment, not the Zulip cloud service (which is still running for existing Zulip users today).
I think my blog post covers pretty well why we open sourced Zulip: https://blogs.dropbox.com/tech/2015/09/open-sourcing-zulip-a....