once worked for a start-up that went through a high amount of churn and employee turnover. one of the pain points was the know-how being locked inside Slack threads and we had hit the 10K message limit months before I joined. The place was also politically toxic and the CEO was mostly the cause of this. Initially when I had still some passion left I suggested to move Slack to a self-hosted Zulip installation (threaded topics FTW) because the CEO constantly complained about having to pay for Slack subscription and he was totally against this as somebody who believes in FOSS.

After getting this Zulip migration approved the CEO pulled the plug in the last minute because he realized during a discussion about how to handle the import of the original messages - that all the old (toxic) discussions would now be in the hands of his internal employees and they couldn't be trusted not reading all the shit him and everyone else said behind each others back.

This made me aware that Slack has some interesting reasons for why teams are locked into their SaaS platform which may have nothing to do with scalability or uptime. In our case it was fear of libel lawsuits and further turnover. While you might be able to live with the insider-threat at SlackHQ with them being able to read your messages, sometimes the idea that anyone in your IT can read everything management has said shared or discussed in the past may be too risky for most.