Using the Internet without IPv4 connectivity

ipv6 only machine still reaches ipv4 sites because dns64 upstream is just faking AAAA records ,makes it look like everything is native ipv6. this part of the trick is happening somewhere else which's not controllable. if dns64 breaks or stops doing the mapping properly then this might break

In IPv4 if your NAT or your ISPs CGNAT stops mapping everything breaks.

Having a stateful mapping device inline in the network sucks for reliability in general. Native IPv6 removes the requirement.

DNS64 exists upstream in your ISP in the same way that CGNAT, does, in a central gateway someone along your rout path. If your CGNAT breaks, it's possible that was also your DNS64 fallback provider. For many ISPs, if you are using CGNAT still for IPv4, it probably means that they haven't even invested in DNS64+NAT64, because you can force devices to be IPv6-only and especially with most consumer devices entirely replace a CGNAT with DNS654+NAT64 today, and it is probably cheaper to do so.