Socket activation is nice for certain things, like webservers. Unfortunately for other services socket activation can cause you far more headaches. For instance, Docker's default configuration used to be socket activated under systemd -- this was changed because it meant on-reboot that you wouldn't have any containers running until someone tried to do an administrative command (that used docker.sock).

Socket activation is more meant to allow for on-demand starting of services without losing connections -- and the design shows that this is the main purpose. For many services, "having a connection" doesn't mean the service is ready yet.

I think that having a notification system for a service to tell the init system it is ready would be nice, though the "obvious" way of doing it (rt signals) wouldn't work for unprivileged services.