Personally, despite their declared mission, I see Slack as replacing phone calls rather than emails. It suffers the same problems than phone : people writing fast without thinking twice about what they say to have a chance to put their idea in the discussion, lost forever knowledge because searching slack is a mess, short and meaningless interventions in the discussion, etc. It is indeed to me better than phone, because you don't have to reply within 5 secs when someone tries to reach you, so it's less interruptive.
But for business sensitive discussions where we expect a lot of efficient brain time and valuable discussions, hell no, send me an email.