The billionaire's money is not in some vault. It is invested and being loaned as capital to other people trying to start companies. Since our system relies on capital, that money will create a lot more jobs being in their hands. Not many factory workers fund the next round of startups.
"The ethical argument would be that tax burden should be greater on those with a larger capacity to pay."
Why? Government is a service we have agreed to pay for. It has no divinity beyond a contract. The same rate is fair and just, but different rates or caps on earnings aren't. If I have ten dimes and tell a child 2 go to government as taxes, then show the child 10 $1 bills and ask them same question, I will get a fairer and better answer than most progressives. We the people give power to government and stealing more from successful people is wrong.
I can see a base deduction to protect the poor, but the tax code now and any system that takes huge percentages from higher income is just wrong.