MIME Quoted-Printable was designed to be a work-around-damage
solution at the time, and was not design to remove the need for
handling 8-bit data in SMTP.
I'd put this somewhat differently - QP was designed to be deployable
in the near-term without first requiring MTAs, etc. to upgrade.
If MIME had required that all SMTP servers be upgraded before sending
MIME messages, then MIME would have failed.
So, my only point is that it appears that some work needs to be done
in the SMTP standards, and beating up on the MIME WG does not compute
given this observation.
I don't think it would be controversial to write a BCP declaring that
SMTP MTAs should support transparent relaying of all 8bit non-control
characters in the message body. Neither would it change much - because
most existing MTAs already provide this.
Putting UTF-8 text in the message header is a different problem entirely.