I don't know either very well (and guess you know phabricator very well) but it seems to me the criterion that there's a big difference between the two on is th

I don't know either very well (and guess you know phabricator very well) but it seems to me the criterion that there's a big difference between the two on is the last -- significant developer community not employed by the leading entity, though it isn't _that_ strong of a point for Zulip; it isn't say PostgreSQL. That also may be the best case for the "will" in the title, though it'd of course be better to say "Why it's relatively likely Zulip will stand the test of time" or "How we've tried to build Zulip to stand the test of time" rather than "Why Zulip will stand the test of time".