Well, I don't know in which world you live, but in mine Outlook is alive and kicking, that's where purchase orders are confirmed, meetings with suppliers are organized, business decisions are made … and in the few instances where it's not going over Outlook, it's because it's on a GApps tenant, in which case it's GMail instead.

The biggest threat to outlook and email today is probably MSTeams, and ironically enough, it federates just fine so that two employees from two distinct companies can chat as if they were on the same tenant.