Yes, but that is the exceptional case. Normally, what the sender
should be what the sender gets. Ultimately, if the replier wants to
something special, then he can do it, but that is not the commn case
therefore it should not be what happens if the replier just presses
the buttons on offer. If the replier wants to do such special
let him cut and paste.
I think you have it sort of backwards. The exceptional case is
where the sender asks for something different than "reply all". The
recipient's MUA should take pains to make the recipient aware that
the sender has requested a different kind of reply, while still giving
the recipient the ability to easily choose between what the sender
asked for, the normal "reply all" behavior, or something else.
And "cut and paste" is just too cumbersome.
No, that just illustrates the problem we are discussing. Approximately
of users (or slightly less by my counting) are happy for repliers to
"reply all". But you must not let the fact that you yourself are in
50% blind you to the fact that the other 50% (or slightly more by my
counting) are vehemently opposed to any default action that results in
their receiving duplicate copies.
a) I don't have any reason to have confidence that your 50% figure is
representative of the preferences of Internet email users.
b) you are conflating at least two things here - one being whether
people want to get duplicate copies of a message sent to a particular
address (IMHO it's fairly safe to assume that most do not want
duplicate copies), another being whether senders prefer that replies
reach everyone who received a copy of the original message - or at
least, don't mind having this as a default.