Still, I have a hard time understanding how this system is different from inetd for socket based servers. Running your sshd server via xinetd rather than systemd has been possible for ages (that was one of the examples in the original LWN article about this, I believe).
And as for cups, isn't one of its "features" (that I don't use) to announce itself to the world (and all those avahi services) as soon as the computer runs? In that case cups needs to be active anyway and not only when I actually try to access it. How is that taken care of?