NAT is also expensive to operate at the scale and reliability that large network operators need. If stateful NAT middleboxes go down, everything goes down -- it

NAT is also expensive to operate at the scale and reliability that large network operators need. If stateful NAT middleboxes go down, everything goes down -- it's a point of failure that's hard to route around! Also there's the issue that you'll end up having multiple devices with the same RFC1918 addresses, which is an immense pain in the arse to deal with.