The /64 boundary is the standard boundary for each LAN. An "end site" (that is, a customer of an ISP) was originally intended to receive one /48 each, so enough

The /64 boundary is the standard boundary for each LAN. An "end site" (that is, a customer of an ISP) was originally intended to receive one /48 each, so enough space for 65536 /64 LANs. You have to forget the idea that addresses are scarce. They were with IPv4, with IPv6, they are not. The whole point of the design is to make sure they are not scarce. Never, ever, anywhere. Conserving address space is not a virtue. Making sure that everyone has addresses available whenever they need some, with minimal delay and minimal administrative overhead, and with little fragmentation in routing tables, that is the goal of IPv6. And there is no real risk that this will lead to address exhaustion, there is enough space for about 40000 /48 per person currently alive.