That's one ugly hack, which assumes (1) WAN1 has static ipv6 (the typical SME has dynamic DHCPv6 address...) (2) all the devices will behave correctly when running on NPT on failover WAN2. Many devices do not know about NPT which is basically NAT for ipv6, and break on p2p protocols like voice, video, streaming. They'll send the wrong NPT address to the other side, which try to connect back to the WAN1 address, which is down because of failover.
It is a hack, no argument. It seems fine for web traffic... You'd have to do some scripting to handle the dynamic prefixes. My own dynamic v6 prefix hasn't changed in years.
If you want "real" failover, get an ASN, your own prefixes, and run BGP. I know that's not for everyone!