Younger generations seem to use various other forms of "inboxing" (messaging apps, FB, etc) already. We can see some of the business documents now just being shared over the cloud, more communication over Skype and just meeting invites over email. As for personal mail, form my experience, the only email I receive today are newsletters, notifications, and the occasional BCCed broadcast mail (wedding invites, etc).

I think email is still relevant only for a document trail for business. If its about leaving a document trail, maybe there should be something new and more suited for it - for approvals, discussions, etc.

Its not like people are looking for a solution for a "problem", but then someone is going to create that something - which will make email less relevant even for the business use.