So first, why it's important for me that we're able to trade continuously. Suppose I'm an HFT who trades the S&P 500 ETF. I put out a buy order at 217.75 and a sell order at 217.76.

If a trader comes in an buys the ETF from me at 217.76 then I need to be able to hedge immediately in other products. Without continuous trading I would need to quote wider.

Second, HFTs make less money per dollar traded than the people who used to do this. In a sense, HFTs "won" because they were willing to do this cheaper than anyone else.

This is different from investment banking, where it's my impression that they get paid these huge fees because CEOs hire their friends and pay them with shareholders' money.

Lay out the case for the payouts being obscene.

Probably keep in mind that the nominal value of the stocks changing hands on a given day is hundreds of billions of dollars and that an actively trading counterparty is facilitating the movement of that money. What's fair compensation for that?