What is interesting to me is that Slack sees themselves as a competitor to email. I feel similarly about Matrix, and I hope that the Matrix protocol does eventually gain the wide adoption seen by email. Unfortunately Slack lacks a lot of features that are necessary for it to compete with email on a wide scale:
* end-to-end encryption support (this is the main value add for email competitors)
* end-to-end encryption support (this is the main value add for email competitors)
* support for federation and self-hosting
* multiple independent client implementations