I guess that depends? The company I mentioned in another thread had a single IRC node that basically never died.

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.