A 20 year old e-mail program is just as useful as a new one, while a 20 year old browser will probably crash once you try to open google.com.
I admit I lose track of time regularly, but a 20 year old e-mail program -- Eudora, Elm, what have you -- had no ability to parse HTML e-mail. I spend most of my time checking my e-mail with my phone and occasionally with a web browser and I use a desktop client every other month or so. I did find myself inspecting the source a few times last month, and I did have a few e-mails without the plain text equivalent. My conclusion was that there are e-mail clients or companies who send HTML only e-mails. This means old e-mail clients from 20 years ago would not be useful.
Email is changing, but far more slowly.