Are you really suggesting that email should just be for work, and that we should ignore personal use cases: photo and link sharing, for example, or just writing a thoughtful message to a seldom-seen friend? Lots of Gmail users do those things, and I reject the notion that email providers should just cede that ground to proprietary communication silos.
The point that I was trying to get across in that article was not that all messages should be shorter. Rather, that the form of the composition UI sets expectations for what the message itself should look like. The form of Gmail's old compose set the expectation that it was a formal medium; that writing something short might not be appropriate, and that writing an email should be like writing a memo in a word processor. That makes messages feel less immediate, less personal, and as a result raises the bar for sending shorter and/or personal messages.
The goal was not to prevent users from writing long messages, but rather to provide a better balance between the two. Ultimately, though, this isn't a personal vs work issue. Facilitating short messages is arguably even more important in work settings where everybody tends to be inundated by large amounts of legitimate work-related email. There is real value in having a UI that gives users signals that it is ok to write shorter messages.
At the same time, on a 1366x768 browser window I can fit more than 3 paragraphs of lorem ipsem from http://www.lipsum.com/ in the default Gmail compose window before it even starts to scroll. In full screen mode at the same resolution you can fit 5 paragraphs before it starts to scroll. The UI isn't preventing you from writing something longer if that's what you need to do.