It is reasonable to demand time to upgrade, so that there are no
interoperability problems. It is not reasonable to demand that this
7-bit garbage be allowed to remain on the network forever, at the
expense of all future implementors and users.
forever is a long time. and yet, even if all use of quoted-printable
were to cease today, there would still be a need for new MUAs to be
able to read old messages stored on servers for many more years, perhaps
decades.
using raw utf-8 in message headers wouldn't replace 2047. instead,
it would add yet another format that MUAs are expected to parse
and display. it would not reduce complexity, it would add to it.
If we add new MIME headers and ESMTP commands, MUA and MTA should be able to
have a smooth transition from 7bit encoded 8bits to UTF8. If there is a
requirement of change anyway, why not change it to something that is more
simple and well support by the Unicode Consortium.