The designers didn't imagine the prefix to be eternal, that's why there's a lot of support for automatic renumbering (address lifetimes with concurrent deprecat

The designers didn't imagine the prefix to be eternal, that's why there's a lot of support for automatic renumbering (address lifetimes with concurrent deprecated and active addresses, with support for this built into address autoconfiguration mechanisms like dhcpv6 and slaac from the start, etc). So it's better than "v4 with more bits".