IPv6 has privacy extensions. On my home network my computer grabs a new IPv6 address every 30 minutes. Yes it's all out of the same /64, but this provides the same sort of smearing that NAT does.
> A lot of people sit behind NAT with many members.
Would it surprise you to learn that some ad networks will actually cross-pollute targeting to the group?
Definitely happens in offices I've worked in.
No, but IP-based tracking is even easier than many other methods and can be done passively.
The IP address may not be the only identifying piece of information, but IPv6 includes strictly more information that can be used to track you, and it makes it far easier to take advantage of that information. Supercookies are bad, but it takes even less tech sophistication and effort to track someone by IP address than with a supercookie.
> Many households already have a single member, so IPv4 wasn't providing much cover there.
A lot of people sit behind NAT with many members. Much as I hate NAT, it does provide this benefit, if only by accident. IPv6 makes it far easier to disambiguate two users behind the same NAT without having to look at any other bits of information (cookies, UA strings, etc.).