And when exactly would they be forced to?
Theoretically, we "could" go on using IPv4 forever, as the main communications protocol, because the existing IPv4 addresses aren't simply going to go away.
It would be pretty painful, and a stopgap, because of CGN and the like.
They're already in use and allocated, and until computers start shipping with no IPv4 network stack it would appear that there is no date in which companies and ISPs are forced to use IPv6.
Right now all we have are a couple voluntary implementations of IPv6, at least in the ISP side, where as pretty much every hosting service (at least the ones I looked at in my search for a VPS) provides IPv6.
It appears servers and websites and hosted services are all ready to make the switch, but when will it be enough to convince the ISPs it's time?
Will it be if computers stop coming with IPv4 network stacks, or when consumer routers ship IPv6-only, or what?