1: I think that's still a decision of the OS developers, you can have a purity-of-install to have functioning software (avoiding incompatible libraries), in the end though for all binaries it's just instructions with little knowledge moving around an instruction pointer, so is there anything fundamental that precludes an overlayed patch-stream where you have installed versions (nix world) but a runtime linker that silently replaces core libraries (that would cause extensive reloads, say OpenSSL) during link-time to have a functioning userland until the next release occurs?
2: I hear you, I still think it's a minor inconvenience in the grand scheme of things, worst case administrators should manage for full user backups in case of breakage (people bad backups habits are more likely to cause havoc for other reasons like hardware failure than the occasional downgrade of system data).
3: only thing that immediately comes to mind is Blender, but there it's Python scripts and intentionally not supporting C++ plugins (last I checked you're supposed to rebuild the entire thing if you want C++ additions).
4: OSS unixy systems has always had tilt to the former in the conflict between the security/sysadmin view (no local binary searching due to security, binaries upgrade everywhere) vs users (who wants to run programs and don't care too much about what glibc version it was compiled with, even taking that big binary with all libraries bundled).
I see Nix as more like Windows SxS instead of just shipping everything like with flatpak,docker,etc. But it always boils down to the same dependency on absolute filepaths for everything that's hard to escape (because you can "just recompile").
flatpak exists though, doesn't it work well enough?