With increased automation, this only gets more extreme.
Taxes on labor are actually a method of extracting money form the rich capital owners.
As you mentioned it's easy for the rich people to hide their wealth and avoid taxes on its growth.
The one thing that was very hard for them to avoid or hide was purchasing labor which they had to do to enlarge their wealth. So governments taxed that.
If governments lowered the taxes on labor it wouldn't mean middle class would earn more. It would only result in capital owners paying less for work. They always pay as little as possible and how little a person is willing to work for is the same, tax or no tax. Because money in hand is what counts.
Of course since as labor is being replaced with automation this way of collecting tax on capital growth becomes less and less feasible, so things are bound to change.
Or perhaps this could serve as a kind of test: a technology that cannot be reliably used in tax evasion enforcement simply isn’t worthy of the name AI.
Or perhaps it reveals that we have structural problems, and certain concentrations of wealth with or without automation are a threat to the just and effective operation of society and should therefore be as vigorously opposed as crime or foreign attacks.
So somewhere along the line it could be very beneficial to be poor on paper.
Or are we going to blame these people for corruption while the (ultra) rich are doing this constantly?
"It's good to be the king!" (Mel Brooks)