The times I've seen a Slack transition, email usage fell by 80% or more. With my current employer, Slack is used for almost everything in team. Email has become more about conversations with external people. Except the ones that we interact with enough that they're now in our Slack.

At this point most my emails are automated responses from companies. Slack on the other hand is people, that suggests that people want conversations that are more nuanced that just back and forth. Email certainly has its place, but it's not right for groups of people talking to each other.

Work e-mails for me have always had a high noise to signal ratio. Daily reports that I can't opt out of, alerts, team e-mails. I only check my e-mail once a day now.

I don't like the randomization of that chat can bring, but being able to join and leave channels is much preferable to e-mail.

More annoying and adrenaline-driven than email, but also more plugin-friendly

Slack integratons are not full-featured but they're more flexible than what you can embed in an email, even with gmail's markup actions. There's no other 'inbox for interactive widgets'.