You can't just assume linear growth. Your link acknowledges this, and the "prediction" you refer to has the right caveat: "we can look at the time series shown in Figure 1 and ask the question: 'If the growth trend of IPv6 adoption continues at its current rate, how long will it take for every device to be IPv6 capable?'". It's using such a naïve model to illustrate that we're still nowhere close to completing the transition, not to say that we'll be done in 20 years at most as you suggest.

Adoption will slow down and my guess is that we'll be stuck with a long tail forever, and only time will tell if that happens at 50% adoption or 90% adoption.