Providers would just do Carrier-grade NAT (as they do today) or another wonky solution with a tunnel into different networks as needed. IPv6 is still useful in different circumstances, particularly creating larger private networks. They could basically reimplement WireGuard, with the VPN software doubling as IPv6 router and interface provider. I'm not saying this is a great idea, but it is definitely what someone today would have done (with HTTP as the transport method) if IPv6 didn't exist.