This is a tale SaaS vendors (which have strong presence in online tech communities like HN because they are software companies) sold very well, and it's probably true for many small startups, but for medium sized companies managing their own platform for something like Slack is completely doable and you will not have those big downtimes compared to Slack. Sure, you have to dedicate time and resources to it, and obviously is not "core business" although a chat platform is a pretty important component in an online company.