If you're a decent sized organization, you can pay $12.50 per user per month to Slack for a messaging service, or you can pay $20 per user per month to Microsoft for Outlook, PowerPoint, Word, Excel, Access, Publisher, Exchange, Sharepoint, and a messaging service. If you just want messaging then you can pay $8 per user per month, and they'll throw in Exchange, Sharepoint, and a terabyte of cloud storage per user on top of it.
I don't think it's possible for Slack to be better enough to justify that kind of a price difference.