In what world is it going to be "the same or even less money" to hire one or two sysadmins on a full-time basis than paying for a Slack subscription?
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.
30 years ago, efnet was (apparently) 35k users.