> but instead to create a bespoke mechanism involving a bunch of tiny files, all of which is insanely system (linux) specific, and therefore completely non-portable.

To nit pick slightly, it's not really a bespoke mechanism it's just re-using the mechanisms provided by systemd. Quadlets are implemented as a systemd generator in order to re-use the existing service management system that exists on essentially all major Linux distros. Quadlets are less a direct competitor with compose (hence why Podman implements the compose spec) and more a way to better integrate containers with the rest of a system. The closer Podman native equivalent to compose is Kube files.