In what way does HFT "discourage investor participation"? Before there was high-speed electronic trading, virtually no investors were out there trying to outcompete market makers or run elaborate pairs trading strategies.
I'm sure HFT does dissuade a class of day traders from participating in the market. But who cares? Electronic trading also killed the human market makers, who simply got outcompeted. The result, again, was an unalloyed good: the human market makers were crooked as a wallet full of 3 dollar bills.