We have no lack of address space in the 32 bit IPv4 world. Half of the old A
space is reserved and so is most of the old C space and above. We exhausted
the old B space and created a limited address space crisis, but CIDR solved
that.
What we are constrained on is routing table space.
So it's not whether IANA gets the addresses back, it's whether ISPs have to
carry all the routes for all the cruft and leftovers or whether they can
safely delist abandoned address blocks. Should be straightforward to do that
based on traffic measurements.
But don't assume that just because routes don't appear to have traffic that
the address blocks aren't being used. They may have disappeared behind a
firewall, where they are still being used. In which case they could safely
be delisted from the routing table or safely aggregated.
--Kent