Iljitsch van Beijnum has written an excellent article on the upcoming doom that will be the IPv4 to v6 transition.
Legacy problems will be huge – there are home users running every ancient OS ever available out there.
There is no plan B. Despite the long list of the issues with IPv6 and its deployment, there are no alternatives. It took us the better part of two decades to get this far with IPv6, and there’s no way we can come up with, implement, and deploy an alternative before the lack of IPv4 addresses becomes a serious problem.