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.
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.