You're not "wasting" them if you just don't need the extra bits
We are talking about addresses, not counters. An inherently hierarchical one at that (i.e. it goes from top to bottom using up all bits). If you don't use the bits you are actually wasting them.
you can gradually make the other bits available in the form of more octets
So why didn't we make other bits available for IPv4 gradually? Yeah, same issue as that: Forwards compatibility. If you meant that this "IPv5" standard should specify compulsory 64-bit support from the very beginning, then why are you arbitrarily restricting the use of some bits in the first place?
If you're worried about wasting registers it makes even less sense to switch from a 32-bit addressing space to a 128-bit one in one go