I'm using slack in my office (We also use trello for checklist but Slack for communication) And I do think it'ss a bad decision for Slack to get into the email world.

It will open a door for security breaches and in my company we have some pretty important content on Slack

I also don't want EVERYTHING to be in one place I like the Slack as it is now, Adding emails to slack aswell will make it a really large platform.