It's also about ports. If I have 10 Gbps of IPv4 flow but my devices are licensed for and operate on 1 Gbps, I need to license 10 1 Gbps ports.
When I was last working at an ISP, various CG-NAT solutions charged not just based on the connection table, but also there was various licensing for various slide-in cards.
If the ISP is having to buy additional hardware/additional ports on upstream providers just to power their CG-NAT that is an additional cost.
If the POP where you have your CG-NAT doesn't have more bandwidth available you end up running your ports hot... so now you either need to rent a new location, get your connectivity up there and start figuring out how to route the traffic.
It's not as simple, and it all costs copious amounts of money. Especially if your IPv6 can more easily be distributed across multiple POP's with multiple forms of connectivity and traffic shaped using BGP or other solutions thereby reducing the load on a single upstream port.
When I was last working at an ISP, various CG-NAT solutions charged not just based on the connection table, but also there was various licensing for various slide-in cards.
If the ISP is having to buy additional hardware/additional ports on upstream providers just to power their CG-NAT that is an additional cost.
If the POP where you have your CG-NAT doesn't have more bandwidth available you end up running your ports hot... so now you either need to rent a new location, get your connectivity up there and start figuring out how to route the traffic.
It's not as simple, and it all costs copious amounts of money. Especially if your IPv6 can more easily be distributed across multiple POP's with multiple forms of connectivity and traffic shaped using BGP or other solutions thereby reducing the load on a single upstream port.