RE: "whether the global keymap C-x 4 will be replaced by a command,"

[Drew Adams]

Read your post quickly.  Apologies if this is
off the mark.

Would the use of the fallback behavior be optional,
i.e., choosable by users (e.g. opt-in)?

Would it affect command (and key) discovery, e.g.,
giving a (false) impression that such-and-such an
other-* behavior, for which there is no explicit
command and no explicit key binding, but which is
available only via your fallback, is in fact a
real command/key?

Would a user be able to somehow reify/manifest
the fallback behavior as an actual other-*
command, which s?he could then bind to a key?

Generally, is the fallback thingy only a plus, as
opposed to being a possible substitute for what
we do now?

It bothers me that you might see it as an eventual
replacement, since it doesn't seem to offer the
same things.  Speaking of "backward compatible"
doesn't inspire confidence that it would be only
an optional plus, and not, e.g., a trojan horse.