That's a bit over simplifying it. Assume you're a day trader and sell your stock to make a profit while on the other side is Warren Buffett who buys from you. It's a question of investment time horizon. Both can be winners here.

Yes, I think day trading is zero-sum

I think gains on assets held for less than (say) two days should be taxed at a rate higher than normal income. On a related note, long-term investment is the good part of capitalism, and I'm ok with taxing gains on assets held for long terms (say 5 years) at a rate lower than normal income. The point is to incentivize people to work hard to find those investments that will pay off in the long run. Implementing the taxes described above would often increase bid-ask spreads; the size of the bid-ask spread could be an indicator of the uncertainty in the market.

Life is zero sum. Doesn’t mean you can't have huge success.

A bit fluffy of a piece.

>Unlike trading money for food at the supermarket where you and the supermarket both know for certain you each benefit

There was a boycott movement in Canada for due to price gouging by the grocers and I've seen 100-200% price increases in certain items in the last 4 years where I'm at (while 0% on others). Someone may be benefiting more..

>You wouldn’t wager money on your ability to beat Djokovich in tennis or Lebron James in basketball, why would you wager money on your ability to beat Citadel?

I don't believe this closing statement is accurate or relevant: you are not directly up against anyone except for yourself. Your strategy and decisions will lead you to profit or peril; against, and just like everyone else.