We are talking about addresses, not counters. An inherently hierarchical one at that. If you don’t use the bits you are actually wasting them.
Bullshit.
I have a 64-bit computer, it can address up to 18.4 exabytes, but my computer only has 32GB, so I will never use the vast majority that address space. Am I "wasting" it?
All the 128 bits are used in IPv6. ;)
Yes they are all "used" but you don't need them. We are not using 2^128 ip addresses in the world. In your own terminology: you are using 4 registers for a 2 register problem. That is much more wasteful in terms of hardware than using 40 bits to represent an ip address and wasting 24 bits.