Jumping in against my better judgment ...
The /64 boundary was for a number of reasons, the fact that "only" autoconfig breaks when that isn't the case is irrelevant (and not entirely true, but many of the breakages are minor/not intractable).
Complaining about it now doesn't help, and many other decisions have since been made that rely on /64s.
So, half-assed or not - this is the protocol we have, and it works today ... So what is the operational debate?
/TJ