Except you can't really replace it. You need to keep BOTH for the good old internet to work.

You probably need some amount of CGNAT or equivalent (e.g. DS-Lite). But if you offload the vast majority of your traffic onto v6 (which is easier than it sounds, because most traffic is to a few big sites and most of them are v6-enabled) then you can reduce the number of connections you have to track by orders of magnitude, which saves you real money.

Yeah I remember seeing that in a talk about an ISP adding IPv6 support: https://youtu.be/75h4gm7t1oI . They say that 30% of traffic is IPv6, which means 30% less CGNAT hardware needed.