The registries (although I shouldn't speak for all of them) would be
more than happy to allocate address space in whatever way the ISPs
agree is the "right way". The problem is, you can't agree on what's
the "right way" of doing it.
It's like herding cats without the benefit of a laser pointer.
They do not want to be tied to their upstream ISP.
Understandable. Hence, the people who actually develop
technology to renumber ISPs and things downstream from
them, whether that be with NATs (hi cisco) or things
emanating out of PIER, will be heros. (Hopefully rich too).
But consider the hundreds of ISPs that don't.
What's NANOG? Sigh.