IPv6 depends on IPv4 addressing if you have downstream devices that need to reach the entire internet. If you go to ARIN they have an explicit policy of allocating IPv4 blocks to facilitate IPv6 deployment.

CGNAT is a term that broadly used to refer to carrier-scale solutions to deal with issues with number exhaustion and dual-stack deployments. What most people think "CGNAT" is, is actually NAT44. https://nfware.com/blog/why-cgnat