Meanwhile, cost of trading for normal people like us has gone through the floor. And we're only really looking at the last 15 years when we think about trading costs, but even steeper reductions precede that, and it was all brought about by replacing human market makers --- who are crooked as a barrel of fishhooks --- with automated systems.
Respectfully --- I don't know you, and this isn't a personal comment --- but my guess is that you distrust HFT because you've been told to distrust it. If you do even the most superficial cui bono analysis, you'll see the people most interested in making you believe that are themselves major financial institutions, all of them far larger than the HFTs.
It wasn't HFTs that brought down the economy in '07-'08. It was their adversaries.