I'd rather stick with Slack than trust Microsoft not to stuff it up
Slack isn't a replacement for e-mail, it's a different beast. We use Slack a lot and have all kinds of inter-team channels, and it works really well for short form instant-messages between team members and sharing links to other resources, and the seach isn't bad for finding all the useful code snippets that someone has sent you later. We still use e-mail for all the other stuff that e-mail is a better fit for.
I can see that Microsoft's offer being free is tempting, but Slack is relatively reliable, has decent phone apps, and a good set of features, whereas MS will probably push this hard now, neglect it for a bit, then at arbitrary intervals push out unnecessary new versions that pointlessly screw up all the features you were relying on, like they did with Skype (et al).