I don't see the urgency being a matter of years, due to the fact that most ISPs are adopting NAT's. I guess my problem is, I don't see why ISPs won't just start

I don't see the urgency being a matter of years, due to the fact that most ISPs are adopting NAT's. I guess my problem is, I don't see why ISPs won't just start deploying NAT's to convert IPv4 to IPv6. I mean with the span of IPv6 addresses, why can't they pin an IPv6 to an IPv4?