The real issue here is running out of IPv4s.
We don't really need more IPs for clients - NAT fixes that problem (I just argue that SOCKS could be a better solution, that can work alongside NAT).
We need more IPs for servers. The IPv6 solution is to move clients and servers to IPv6. 6in4 gets around needing to migrate the ISP as well, which is good, but it's still a heap of work that has to be done by people who don't get any material benefit out of it for years, at best. So it won't happen.
There is indeed a scaling issue with NAT/SOCKS as both need connection state on the gateway device - but, I suspect, the dropping cost of hardware versus the rising cost of IPv4s will favour more hardware. The real question is how it'll stack up against the cost of dropping IPv4 for IPv6 (not the cost of adding IPv6 support alongside IPv4, which isn't that hard).
