i don't have a subscription to laundry detergent, nothing stops me from buying a different brand in 6 months, whereas slack has contracts with my company, and has switching costs (we'd lose our message history).
You can't export your data from Slack?
You can. Nicely it can be had as json. But it's really only half useful since there are links and attachments that point within Slack itself and that content is not captured in the export without additional processing.
Email is non-sticky (just take your emails/save them on client and leave) Files storage is non-sticky (just move your files from OneDrive to GDrive) Chat/Collaboration is sticky as mud.
Maybe you should think about a formal method of documenting important decisions for your company instead of relying on a chat log.
"Hey, I just purchased 20k of widgets"
"What? Why?! We can't afford that! We decided to discontinue those widgets month ago! Didn't you scroll through the chat logs before making that purchase order??!"
Good luck.