CGNAT is not only "IPv4 to IPv4" translation, though it is the most common mode, calling NAT44 (NAT444). CGNAT also allows translating "IPv4 to IPv6" and "IPv6 to IPv4".
Like some people still use Windows 95, there certainly will be
devices, websites, services that do not support IPv6.
We need these types of translation because even if an ISP wants to migrate to a native IPv6, it must support IPv4 for many years. Like some people still use Windows 95, there certainly will be devices, websites, services that do not support IPv6.