The fact is tax increases of any kind are currently hard to get. Maybe as benefits for seniors start disappearing we'll see that change since seniors vote. If we ever get back to 20% of GDP for Federal tax revenues there's no special reasoning that says getting to 21% is going to be any harder then going from 18% to 19%. The whole 20% thing is just silly.
It doesn't show any such thing. The whole truth is that from 1940 to 2000 average taxes paid including state and local went from 18% to 32%. Plenty of governments around the world and a number of US states are over 20% in federal taxes alone.
"Tax avoidance behavior increases as tax rates increase, you have moving targets."
This is obvious but, unfortunately, does not get you to "it's impossible to collect more then 20% federal taxes"