The reason email usage is so ubiquitous is because everyone has an email address, and the protocols are well-known and universal.

How else would you propose inviting people to a meeting? Put a paper memo in their mailbox asking for a written reply? Call everyone individually on the phone? Walk around the office and gather everyone together?

If you're thinking about the meeting problem, do you propose a meeting/calendar app/client or protocol that is separate from email? Then the problem becomes saturating your user community with your new client. What do you do if someone doesn't use your new meeting/calendar app/client?

I think what you are describing is a problem with meeting/calendars not email.