I don't think Slack could universally replace email, but it could for some teams (ubiquitous addresses aside). One of the teams I work with communicate solely

I don't think Slack could universally replace email, but it could for some teams (ubiquitous addresses aside). One of the teams I work with communicate solely through Slack (and do use it asynchronously). All the other teams use some combination of email and Slack, and it wouldn't surprise me if there were teams out there avoiding Slack altogether. It's just another tool. The usage depends entirely on the audience.