If something needs to be permanent, you should create a real document and publish it to a shared repository. That way knowledge, history and commentary is kept in a single place and not distributed throughout everyone's inboxes.
"Should" is the keyword here. Not that I disagree with you, but nobody is actually going to do this. Ever. Email is too easy to be replaced by document-creation procedures.
Also, email is great for showing record of communication. It's subject-based (except for the idiots who reply to an email with contents that have nothing to do with the subject) so it can be easily categorized and remembered. "Where was that email Rick sent? Oh yeah it was around the beginning of Nov, and the subject had 'wombats' in it." Sometimes you need a subject-based list of records.
I really think anything people end up "inventing" to replace email will wind up being the exact same thing as email, but with a less distributed architecture.
That's an awful excuse. People rely on libraries in order to do their jobs. A "worse" library with better documentation helps more people.