The one thing I like better about Slack over email is the lack of constant spam from random apps.
Working in a company means that only 1% of my email is actually from humans. I must constantly add new filters, because even small companies today use dozens of SaaS products that are constantly fighting for my attention, and only a small fraction of those messages is actually useful for me.
Slack so far is just humans. Plus integrations mostly added by developers for developers.
Of course this can change at any time, and I'll have random robot spam in Slack. Which means I will have no option other than start ignoring Slack as well and start asking people to come to my desk. Which might cause me to finally go into carpentry as I have been promising to do for the last 15 years.