FWIW, the common rebuttal scenario goes like this:

You and I earn $200k in wages, and each pay $100k in taxes. You spend your remaining $100k on a boat. I invest my $100k, and 5 years later it's worth $200k. Suppose that $100k gain is taxed like ordinary income at 50%. After taxes I have $150k, which I also spend on a boat.

Without taxes, you would have had $200k to consume in year 0, but you actually got $100k; your effective tax rate is 50%. Without taxes, I would have had $400k to consume in year 5, but I actually got $150k; my effective tax rate is 62.5%.

Who should pay the higher tax rate? The person who consumes their wage income immediately, or the one who defers consumption? For them to be equal, the capital gains tax rate has to be 0%.