ISPs are also making it more difficult to themselves: if there is a support for IPv6 at all, it is DS-lite, not full dual stack.

So let's say I'm willing to migrate to IPv6, but I still need IPv4 for some reason (maybe I need to do the migration with multiple independent ISPs and every single of them is traveling at different speed wrt IPv6). This approach makes it impossible for me to switch where it would be possible, I cannot drop IPv4 yet, CGNAT-ed connection is unusable, but also I cannot use IPv6 where I otherwise could.

So I stay with IPv4.