Well...
> Tests show it's just too hard to put the unused 240/4 block to work
Probably easier than deploying IPv6
> Two thirds of the internet is not on IPv6 and is thriving on network address translation
Exactly.
IPv6 reminds me of OSI networking, as beloved by mainframes of a certain flavour. Designed by committee, and including all sorts of clever stuff to satisfy everyone. Unfortunately they both ended up being too hard to implement, and as a result, the simplicity of IPv4 still rules the internet.
(Yes, I've deployed IPv6, and until recently had a fully IPv6 enabled network - DNS, email, web, etc. Unfortunately the problems it causes (several*) seem to exceed the benefits it gives (none), so I'm now removing all IPv6 capability - I just don't need it.
<asbestos underwear installed...>
* For starters:
Problem 1: The IPv6 internet seems to be fragmented - not all addresses are always reachable from all providers due to commercial politics.
Problem 2: If I change provider, I have to change addresses on everything. Yes, I could use IPv6 NAT, but that was strongly discouraged by the IPv6 cult and only got included (late in the day) because it's necessary in the real world.
Problem 3: Practically no consumer ISPs (in the UK at least) support IPv6 properly (i.e. give you a /48 global prefix)