"Slack would insist that email's time has passed"
Well they would, wouldn't they - but as a victim forced to use Slack I can say I'd MUCH rather be using Outlook.
Slack's the communication equivalent of running into a room, shouting a message, then running out. Great if you're in the room at the time and hear it but if you weren't and it's stopped echoing your chances of knowing it happened are next to zero!
I spend half my time having:
"Didn't you see X, we posted it on Slack?"
"Which channel?"
"The Y channel."
"We have a Y channel?!?"
"HR" keep pulling this trick, changing rules/policies/processes and only informing people on Slack - but I have no idea what out "HR" department's actually called this week, if it's HR, People Operations, Human Capital Management - and even if I guess that it doesn't mean the same name's been used in the one of 10,000 channels they are currently using.