I'm certainly no expert whatsoever. This is just my understanding from talking with a few folks I consider quite smart who work in the space. Some working for HFT firms, some elsewhere. Also reading on the topic over the years.

There does seem to at least be some evidence that HFT firms decrease retail spreads overall. Either way, my main point being made is that negative impact to retail traders is very much in question.

https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2183806

That seems to be about a fee change that increased costs for market makers, widening spreads.