If we actually get to the point of address shortages,

Either, NATv6 would become a thing, or instead I hope SLAAC would get deprecated and dhcpv6 would become mandatory so we could give out smaller than /64s

2^64 is 18,446,744,073,709,551,616. That's 18 quintillion. 10^19. There are ~10^10 people on the planet. Each person could have a 10^9 networks (not even devices) before we ran out of /64s.