That wouldn’t have any real effect. Think about it with a simple exaggeration.

Joe Public buys a stock for $10 and sells it a month later for $30. Let’s say that hypothetically he had to pay a new high tax on his capital gain of 50%. So he’s paid $10 of his $20 profit along with $6.95 of trading fees. Joe Public now only has $3.05 left of his profit.

Goldman Sachs buys 1,000,000 shares at $10 each and sells them 30 seconds later at $10.02 for a profit of $20,000. They pay the new high tax of 50% and have $10,000 left (remember, they’re direct connect brokers so they’re not paying trading fees of any significance). So, they still made $10,000 in 30 seconds thanks to just one HFT that’s going to continue executing for the entire trading day. 80% of all trades are now these HFTs.

Sure, it’s not $15,000 anymore by this example. But who’s really hurt? The general public. Not the bank.