Thanks for taking the time to do that!
Lets say 20% adoption means we're 40% of the way through the transition. The slope for IPv6 looks better and better every day, but overall it's not a great adoption story for tech that inevitably has to happen. (not to discourage or minimize the all the hard work done in getting IPv6 this far)
I dove into the world of curve fitting (wee!) and my prediction[1] for 95% IPv6 adoption is around the year 2025: https://imgur.com/a/LyBJn (fitted to the logistic curve[2], x=0 is basically 2010, y is percent adoption)
[1] Which you should completely trust because I've been doing this for all of 20 minutes!
[2] https://en.wikipedia.org/wiki/Logistic_function#In_economics...