In fact, 822 was a _simplification_ of its predecessor, 733.  And the reason 733 was so complicated was that there was no earlier standard at all (680 existed but was never widely adopted) and pretty much every mailer went its own way when it came to headers.  733 was therefore designed to be all-inclusive, so that both "From: John Cowan <xxxxxx@ccil.org>" and "From: xxxxxx@ccil.org (John Cowan)" are compliant (and are to this day).  It wasn't a matter of insufficient extensibility; indeed, 822 was extensible enough to be adopted with variations first for Usenet news and then for HTTP.  It was a matter of gutless standardization.