I think I should clarify some of the stuff you're saying:

1. Zulip has been in development since August 2012 (1 year before Slack was released), so is not trying to be a better Slack.

2. Anyone can host their own Zulip server, and have control for all their data.

3. For anyone wanting to migrate from the hosted instance to on premises (self-install) can easily get an export of all their data that they can directly import into their other instance.

4. This is anecdotal but so far, Zulip's full text search has worked well in finding messages from over an year ago easily.