(or better yet, don't use Atlassian, but that argument's a non-starter at my company).
Operating it is a mess too, it's close to a full time job, I can't imagine recommending to host it like I do with communication software.
For 8 years (the life of the company) that machine sat by and did it's job without problems.
Was it patched? No, and that's annoying, but that cost literally _nothing_ to the company (except a sip of power).
Zulip isn't bad, it definitely doesn't need a whole person to manage it, probably 1hr/mo is good enough.
How much you pay for an hour (or even 10!) a month of engineering time is almost certainly less than you'd be paying for Slack if you're more than 100 people; especially at their single-sign-on provider pricing.
Also you: “but that cost literally _nothing_ to the company.”
Risk is an expense.
At what cost? Even with the occasional outages, Slack is still more cost effective than having your own person (or team for larger organizations) handle your communication software.