SLAAC-by-default is not, in my experience implementing IT automation, an actual barrier for adopters. You have a router sending out RS instead of a DHCP server, to an admin this is not a meaningful change.
You don't exactly need a translation layer. If they just gave me 1.1.1.1:: in v6, anyone migrating v4 to v6 would have the same route to me as before, and other changes like DNS6 could be gradual. Then after v4 is abandoned enough, I can sell 1.1.1.1.2:: to someone or use it instead of NAT.
SLAAC is a good design as a non-default that people who know what they're doing could enable, but a lot of people don't even want public v6 addrs for hosts, they just want NAT/DHCP.
Think about this on a concrete, packet by packet level - I, from a v6 network, with a 128-bit address that cannot be represented by IPv4, decide to open a connection to 1.1.1.1. 1.1.1.1 doesn't have v6 set up, and so can't read my v6 packet and craft a response packet, because its address is invalid in v4. We need a gateway in the middle that will translate the packets from one format to another and perform the NAT function from one address space to another. This is an irreducible complexity.
> a lot of people don't even want public v6 addrs for hosts, they just want NAT/DHCP.
People don't care about whether their address is public or not, they want connectivity. SLAAC gives that to them; it is you that are insisting on adding complexity for the sake of having a non-routable address. As a user, I enable IPv6 on my router and go on my merry way; as an ISP, I assign my customer's router a /60 or /56 via DHCPv6 instead of a /24 and go on my merry way. Running an IPv6 address allocation system is a an easy, solved problem and has been for decades.
The hard problem is dual-stacking, and there is no way around that.
> If they just gave me 1.1.1.1:: in v6, anyone migrating v4 to v6 would have the same route to me as before, and other changes like DNS6 could be gradual
That's not really how routes work. Or DNS.
I think you're mainly arguing we should import v4's allocations into v6, but about the only benefit is that people don't have to bother requesting new allocations. It doesn't help with any other aspect of the transition, and there are good reasons to avoid doing it -- the v4 address space is highly fragmented and also very unfairly allocated.
Plus we have plenty of people saying we should take back v4 allocations from companies that own them. That's not possible, but _not_ giving owners of v4 /8s an entire 1/256th of the v6 address space certainly is.
