Get this: closed circuit television systems actually existed before updates could be delivered by the internet. Another crazy fact: there are still products sold which don't connect to the internet. You don't see nearly as many ads on these products though and don't seem to be as popular these days.

It's not actually hard to do that today with existing equipment: Put it all on a network switch and then just... don't plug it into the Internet. Plenty of on-premise solutions exist and work with or without remote connectivity.