There have been many cases where I have wanted to change the
default "To:" address after hitting "r" (eg someone emailed
me from work but timing/circumstances mean I have to reply
to them at home).
If you think that you will never want to change the default then
you could set $fast_reply
| When set, the initial prompt for recipients and subject are
| skipped when replying to messages, and the initial prompt for
| subject is skipped when forwarding messages.
For me, there are enough times when I do want to change the
"To:" on a reply that it doesn't make sense to set $fast_reply,
because without it all you have to do is hit enter to confirm
the default (and it gives you a extra chance to see if someone
is tricking you by for example setting their "Reply-to:" to go
to your boss instead of them).
The thing about Up Arrow was a red herring really. When Mutt
does give you the option to enter or edit an email address,
history recall of previously entered addresses using up/down
arrows is always an option, along with tab completion, address
book extraction etc. You just didn't want it to give you the
option in the first place.