All trading (up to epsilon error) done by retail and institutional investors is with HFT as a counterparty -- disrupting HFT has direct consequences on the bid-

All trading (up to epsilon error) done by retail and institutional investors is with HFT as a counterparty -- disrupting HFT has direct consequences on the bid-ask spread they're capable of maintaining (in the worst case, they'll have to shut off their market making) and consequently affects the cost of trading for all market participants, not just HFT.