A report from the RPM summit

It sounds like what you want is an 'upgrade' package - an RPM that can be installed over an existing package to update it to the newer version. This is slightly different to the idea of delta RPMs IIUC, but might not be to hard for an interested party to implement given that base work (or might be terribly difficult for all I know, but I wouldn't expect so).

A bigger problem really is that most library authors have very little interest in maintaining binary compatibility, so anything which isn't going to be constantly rebuilt needs to be statically linked, with all the downsides that has.

As I understand it we're moving in the direction of automatic dependency versioning (in Debian at least, don't know about the RPM world), so at least that would mean less arbitrary dependency breakages between packages which really are binary compatible after all (your point 5).