By being IPv6-only they are effectively making their users to preferentially connect over native IPv6 though.

Personal anecdote, but once you have IPv6 setup properly (meaning your devices prefer IPv6 over IPv4) 70-80% of your internet traffic will be IPv6.

The NAT64 is really just there for the holdouts.

That's a bit like saying AC electricity was just a fancy way of delivering what customers really wanted, DC energy.

I'm sure that DC customers used their Edison DC equipment for decades after the grid went AC only; but in the long run the newer, flexible, lower overhead system became the default for new equipment and the compatibility cludges were abandoned.

No; most sites I reach from the phone seem to be reached via IPv6. E.g. hitting whatismyip.org exposes an IPv6 (though mentions an IPv4 because they're trying to discover that, too). Some sites do not support IPv6; for those indeed there's a XLAT464 service.