This sort of "mana from heaven" success that Zulip has been seeing reminds me of the time Google Reader announced that it was closing and Feedly was just sitting there with a product that was exactly like Google Reader and could import your subscriptions from Google Reader with one click.

I wonder how often people succeed with these sorts of business models, which seem to be summarizable as "copy something extremely successful exactly, wait for them to fuck up sufficiently badly, and make sure migrating is really easy."