Yeah, the stock market may be positive sum over the long-term, but it's certainly zero sum over the millisecond-term. Whether it's "retail" or "institutional" that is paying for HFT profits, it's all retail in the end.
The millisecond-term zero sum game is part of what allows for a positive sum long term. For example, zero fee trading was pioneered by Robinhood and only possible because of payment for order flow, and as a result it's virtually unheard of now for retail to be paying per transaction. Now more retail investors can participate and everyone benefits. You can also point to lower spreads and faster execution as direct benefits.
The millisecond-term zero sum game is part of what allows for a positive sum long term. For example, zero fee trading was pioneered by Robinhood and only possible because of payment for order flow, and as a result it's virtually unheard of now for retail to be paying per transaction. Now more retail investors can participate and everyone benefits. You can also point to lower spreads and faster execution as direct benefits.