I can see Old Spock giving advice if Starfleet truly needed it, but I don't see him preemptively taking out every single threat to the Federation. Especially since this is a different timeline and his knowledge only applies to a timeline with ships that are smaller and seem to be less advanced. For all we know the new Enterprise could stop the Doomsday Machine will one shot. It would also limit the Federation and interfere with their development. They need to learn how to stop some threats on their own.