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

It's weird having a subnet size equal to a complete IPv4 Internet worth of IPv6 Internets but I believe the rationale was that you would never in practise run of out IPs in your subnet. A lot of Enterprise IPv4 headaches are managing subnets that are not correctly sized (organic growth, etc.). IPv6 is always routable for the same reason (companies reusing RFC1918 making connecting networks a pain).

There are different headaches with IPv6 - such as re-IPing devices if they move subnet - i.e. move physical location, or during a failover etc.

I'm not sure what the best practise there is as many enterprises don't use IPv6 internally. In my experience anyway.

One is quantitative: you have to remember that 2^48 is a much much bigger number than 2^32. With 2^32 IPv4 addresses, you have about 0.5 addresses per human being on the planet, so right away you can tell that stringent allocation policies will be needed. On the other hand, with 2^48 /48 ranges, there are about 8,000 ranges per human being.

So even if you hand a few /48s out free to literally everyone who asks, the vast majority will still be unallocated. A /48 is only about 0.01% of what could be said to be a "fair" allocation. (And yet, a /48 is so huge in absolute terms that even the vast majority of organizations would never need more than one of them, let alone individuals.)

The other is that unlike, say, the crude oil we pump out of the ground, IP address ranges are a renewable resource. If you hand out a free /48 to every person at birth, then long before you start running out of ranges, people will start dying and you can just reclaim the addresses they were using.

Really, just adding 2 bytes to IPv4 would have fixed everything and made it a lot simpler to move over. IPv6 is overkill and I think that really hurt its adoption. I remember being at uni and being told "this is the next big thing". In 1993. And it's not even a big thing now. Not on the user side anyway, I can still access everything from IPv4.