Changing the license of every article would take a
long time, but it
could be done. Changing a fraction of the articles would be even
easier.
The fraction that have only been edited by logged in and currently
active users, yes.
And that's even if you accept your assertion that
"Changing license
would require the permission of every contributor that hasn't had all
their contributions removed".
Do you question the assertion? The only way you could get away with
not having someone's permission would be fair use, but that causes
problems with reusing parts of articles. While one sentence may be
fair use when used in the entire article, it might not be fair use
when used as part of just that paragraph.
Finally, anonymous contributors don't really have
standing to
challenge most of the problematic parts of the GFDL. They're
anonymous, so they can't sue you for not putting their name on the
title page or in the section entitled history.
I really have no idea how the GFDL interacts with anon users. As far
as I know, the license doesn't mention them. That's one of the biggest
problems with the GFDL for a wiki.