liberate /lib

Yes indeed. Multiarch works, but requires support from the build system (and some don't support relocation of the libdir) and even from applications, if they load plugins (and glib and gtk at least hardwire module loading from subdirectories of "lib", so distros have to patch this away today).

It looks like fat binaries instead of multiarch would pretty much Just Work without requiring such a degree of thought (not that it's *much* thought, but it seems to be too much to expect from e.g. the gtk devs).