You're right, the "Windows XP User on IPv4" is just an allegory of the critical mass of users that will keep us from dropping our IPv4 addresses many years into the future.

Put another way, what is the incremental value to provide access to your service to those IPv4 only users/devices? Whatever that value is, in theory you would be willing to pay a portion of that for access to IPv4 address space.

Luckily supply is not really constrained, so much as it is controlled. You can always get more IPs if you need them, but the cost associated I think will continue to increase... until enough people not only just support IPv6, but actually abandon their IPv4 addresses.

When the only devices that your software or service is designed to run on all support IPv6, then there's "no point" in having an IPv4 address. You almost have to get to the point where IPv4 is "not worth the trouble". And we are very, very far from that point I think. More to the point, more people are likely to think that it's IPv6 that is not worth the trouble.

Look at the other side of the coin though. If you have a service that only talks ipv6, you will be insulated from all the unsupported, unpatched, and trojan/virus laden XP machines for years to come!

My strongest incentive for implementing IPv6 at work is that Gmail is advertising MX hosts that have both IPv4 and IPv6 addresses. So on systems which are IPv6 capable, and where your OS either picks a random IPv4 or IPv6 address or prefers IPv6, you'll end up getting lots of noise when it tries IPv6 and fails and falls back to the next address if you don't have IPv6 connectivity set up.

Could of course just ignore that, but it's a good low pressure reminder to get around to sorting out IPv6...