Email is not just IMAP and SMTP. It's much much more (even thought technically speaking the following doesn't have anything to do with email).

Pretty much the only way to invite someone to a meeting at work is to send invite over email. To figure out the place for your meeting, you use "email" client for that. Because you have not sent emails previously to all participants, you use "email" client to figure out the email address of those persons. And the recipients use their "email" client to update their calendar so they remember to come to the meeting.

So to disrupt email, you need to disrupt calendaring and address books and what ever it is called when you search your co-workers email address by their partial name or some wacky username.

It's a mess and a huge beast.