That takes care of the software and protocol side of things, true, but does it give more reliable and predictable uptime? That's the main thing here; while there are plenty of software alternatives to Slack, their product is not just the software but also the hardware, servers, and scaling. You can get a Slack instance from 10 to >10K members without ever having to worry about your hardware, or how much hours your staff needs to spend on maintaining said hardware. And when there is inevitably downtime, you and your staff don't have to scramble to get it back up - with this outage, it's a shrug, it's down, it'll be back soon probably, I'm going to do some work or do something else. Extended toilet / lunch break.