Slack is just warmed-over IRC
The problems with email, Slack, and every other messaging system are social and behavioral in nature. Moving to a new system, like getting a new email after changing jobs, gives a short respite but dysfunctional collaboration patterns reassert themselves quickly.
Like IRC, texting or chat, Slack is good for throwaway exchanges, but it's not going to solve the problem of efficient work habits. That will probably take one or two generations to happen, after all people are only just starting to take a more systematic approach to handling their to-do lists withGTD et al.