The issue is not NAT as a firewall. Every routers shipped to customers also come with a firewall and replicating what NAT does by default is dead simple. That’s

The issue is not NAT as a firewall. Every routers shipped to customers also come with a firewall and replicating what NAT does by default is dead simple. That’s a red herring. The issue is that migrating to ipv6 meant changing equipments on the backbone and dealing with the change and no operator wanted to bear the cost. It’s getting better now that there is a dearth of ipv4 and people see an immediate benefit.