[zamadatix]

/64 is about a lot more than SLAAC, that's just one easily seen place it shows up. Even without SLAAC the /64iveness of IPv6 would still be there. End client subnets being /64 (with the exception of host routes and p2p networks) allow hardware route scaling to be roughly twice as high, allow network operators to never worry about what the client subnet size is (it's /64 and that's big enough for anything, if it's bigger it's an aggregate route, the address is always split in the half for routed part and client part), and allow various tricks like SLAAC assignment methods to work reliably. A /56 gives 256 subnets (and a /48 65,536), if a house needs more than that for hierarchy then something besides the base /64 is wonky.

Uncooperative ISPs will always exist. If the minimum size was /96 they'd hand that out instead of a /64. If there was no minimum size then they'd hand out a /124 or something equally as stupid. For all of the corporatize ISPs and well planned uses, starting at a /64 makes the rest of life easier.

It's the same logic, though often more accepted in this case, as the minimum advertiseable prefix on the internet being a /48. That also has nothing to do with how many theoretical end clients could fit in that and everything to do with what that actually means to equipment, scaling, and implementation planning.