Tbh., if you need two full-time sysadmins to run your IRC server + bouncer + log backup + web frontend, you're doing something really wrong. It's more like an after-school job for a bored CS student.

I don't know how large your company is, but with 10k+ employees, it's suddenly a reasonable option.

With that many employees you're signing up for Slack's enterprise tier, and you can bet they price it so that they come out the right side of this cost/benefit calculation for a lot of companies.

I have seen those numbers. You could easily afford paying sysadmins with those numbers.

But managing an IRC server cluster for 10.000+ users is no simple task.

Paying experts in their field (chat tools in this case) is most of the time the most reliable and cheapest variant. You must also include that Slack has plenty of features that are not available in IRC and mean the productivity is not as good.

Since when is running any service for a number of people something people compare with running google?

30 years ago, efnet was (apparently) 35k users.