Perhaps we are starting to see an impact from LISP
IPv6 always suffered from three problems:
1. The benefit of switching is largely to the community rather than to the adopter.
2. The lack of a migration plan.
3. Clever engineers thinking up more ways of improving IPv4 address space utilisation.
The "running out of IPv4 addresses" problem was always over-stated because it assumed that sparse utilisation of an address space would always be the norm because the allocation strategy has to be dominated by routing efficiency - doesn't it?
Technologies like MPLS have greatly increased the efficiency of address allocation and now LISP (RFC 6830) is providing a generalised model that allows global IP Addresses to be densely allocated to Hosts or Autonomous Systems while, at the same time, allowing a separate address space to be used for the underlying network - i.e. allocated with topological efficiency in mind.
LISP is IPv4/IPv6 agnostic and works with both. It's actually a good way of running IPv6 end to end over a corporate network that's still IPv4 based. Maybe some are starting to do that - keeping all their IPv4 kit (with a private address space) - but allowing for IPv6 externally - and hence distorting the stats.
However, that may just be the optimistic view. The point is that IPv4's 32-bit address space always could address all the atoms in the universe - it just couldn't also route efficiently to them. Now, with LISP it is possible to densely allocate the 32-bit address space while still having efficient routing.
If LISP had been around 25 years ago when IPv6 was proposed then I doubt whether IPv6 would have got enough support to have got off the ground.
Of course, we are where we are with a mixed equipage. However, it is now the case that if an organisation can't be bothered to move to IPv6 and already has enough IPv4 addresses for its own use (which most do if you can allocate them densely) then LISP gives a very good technical reason to avoid the move for anything other than externally facing systems.
LISP is now part of (e.g.) CISCO's product line and just maybe we are starting to see an impact from this.