Why was NAT64 developed? What does it do that can't be done by deploying IPv6 + NAT444, and allocating your customers a (public) IPv6 and a (private) IPv4 address (behind a shared public IPv4 address)?
One of the drawbacks of NAT444 (pointed out in the comments) is that inbound connections require the customer to get the ISP to somehow map it for them. But NAT64 has the same issue for inbound IPv4 connections (AFAIK) so there is no improvement in this regard.