Oh come on
First, there are indeed lots of ways for people with disabilities to use the internet. I myself am cmpletely blind and my computer uses synthetic speech. As previously mentioned there is voice recognition (amazing how many people reposted the same thing about the injury even after this was mentioned) and on screen keyboards. Also tablet PCs with hand writing recognition.
Secondly, text is far more convenient than voice when you need speed. Quicker to send, easier to edit, and easier to skim through. Also don't kid yourself that government people would use standard set ups to send/receive email, they probably use at least uncommon protocols designed for high security. A list of emails with subjects is far easier to prioritise than voice messages, since (if you're not blind) you can just glance at the sender and subject to see what it is about as well as any possible priority level system in place.
Once upon a time it would mean calling your secretary to see if anyone had left any messages. However the nature of the world is changing rapidly, and like with television and radio you can either embrace it or hide from it. Of course radio and TV was less of a leap because all policicians love the sound of their own voice, and are used to speaking in front of an audience.
As to email being a fad, there is a huge difference between professional communications and social communications. Im and to some extent SMS are more used for social situations, email is just not currently beatable for business communications. It's the new fax, and how many individuals owned fax machines as compared to businesses? It of course also depends on how much the business depends on communications, more likely with business to business focused companies than shops selling to consumers.