so cursor motion commands such as C-f would still not normally
deactivate the mark, except when the mark was activated by
a shifted key. This is the desired behavior.
I doubt that such a broad change is good.
Also, it is rather painful to implement, since there are dozens of
cursor motion commands, if not hundreds. Lots of modes have their
own motion commands.
I think the only reliable way to implement that feature--if we want
it--is to do it in the main loop, after the command returns, if it has
moved point. But that hard-wires the meaning of non-shift, which is
very non-Emacs-y.