> I'm sure we all know the benefits of private networks, so I don't need to reiterate it

That is I think the key. Private networks have sufficient benefit that most places will need one.

The computers and devices on our private network will fall into 3 groups: (1) those that should only communicate within our private network, (2) those that sometimes need to initiate communication with something outside our network but should otherwise have no outside contact, and (3) those that need to respond to communication initiated from something outside our network.

We could run our private network on something other than IP, but then dealing with cases #2 and #3 is likely going to be at least as complicated as the current private IP range approach.

We could use IP but not have private ranges. If we have actual assigned addresses that work from the outside for each device we are then going to have to do something at the router/firewall to keep unwanted outside traffic from reaching the #1 and #2 types of devices.

If we use IP but do not have assigned addresses for each device and did not have the private ranges I'd expect most places would just use someone else's assigned addresses, and use router/firewall rules to block them off from the outside. Most places can probably find someone else's IP range that they are sure contains nothing they will ever need to reach so should be safe to use (e.g., North Korea's ranges would probably work for most US companies). That covers #1, but for #2 and #3 we are going to need NAT.

I think nearly everyone would go for IP over using something other than IP. Nobody misses the days when the printer you wanted to buy only spoke AppleTalk and you were using DECnet.

At some point, when we are in the world where IP is what we have on both the internet and our private networks but we do not have IP ranges reserved for private networks, someone will notice that this would be a lot simpler if we did have such ranges. Routers can then default to blocking those ranges and using NAT to allow outgoing connections. Upstream routers can drop those ranges so even if we misconfigure ours it won't cause problems outside. Home routers can default to one of the private ranges so non-tech people trying to set up a simple home network don't have to deal with all this.

If for some reason IANA didn't step in and assign such ranges my guess is that ISPs would. They would take some range within their allocation, configure their routers to drop traffic using those address, and tell customers to use those on their private networks.