SEND secures NDP by putting a public key into those 64 bits, and also having big sparse networks renders network scanning rather useless at finding vulnerable hosts, so there are reasons to make subnets /64 other than SLAAC.

Also we can always reduce the standard subnet size in 4000::/3 if we ever somehow run out of space in 2000::/3 (and if we don't then we didn't sacrifice anything to use /64s).