[markonen]

Right. Dual stack IPv4 + IPv6 is strictly worse for reliability than plain IPv4 because both stacks need to work perfectly or you'll encounter issues. In that sense it provides the opposite of redundancy.

Happy Eyeballs (RFC 8305) is the typically touted solution, but even that doesn't help in the scenario you describe: IPv6 connectivity wasn't broken.

The problem is simply that we now have twice as many things that can go wrong and no plausible route away from dual stack in sight.