Won't UAs that support raw utf-8 in headers but don't support decoding
2047 tagging in header fields work exactly as "well" as the UAs that
existed when RFC 2047 was first implemented?
no. because legacy UAs could still parse, and correctly reply to,
messages with RFC 2047 (and its predecessors) encoded-words.
Which is exactly what a future, hypothetical, UA that did not
support RFC 2047 encoded-words would do.
but legacy UAs that did support RFC 2047 would not necessarily be able to
parse utf-8.