[tjgq]

Given that worldwide Internet adoption is still growing strong, it is conceivable that sooner or later a significant number of endpoints will only have IPv6 connectivity, or that their IPv4 connectivity will be limited to a private address behind a carrier-grade NAT.

Another point to consider is that there are things you can do with IPv6 connectivity that are very difficult/costly, or outright impossible, in NATted IPv4 land.

Personally, I'm waiting for the next generation of peer-to-peer protocols that make use of end-to-end IPv6 connectivity instead of hole-punching and proxying through third-parties. (Case study: ever thought how ridiculous it is that in 2015 it's still non-trivial to send a large file to someone over the Internet without using some sort of storage service?)

It's plausible that one of those new applications could end up being the "killer app" for IPv6.