I was going to include market makers in my previous post, but didn't to keep things simple. I agree the benefits from them seem pretty clear, in the sense that lowering the spread and increasing liquidity helps retail investors (and maybe market stability in general). But there are a few points here that I think make a lot of people uncomfortable.
First, it's not so clear this is meaningful work. If I'm a smart college graduate and go to work in biotech, I get to save lives and cure disease (ideally). If I go to work at a market making firm, I get to ... lower the spread? I think it's quite reasonable for someone to forgo doing this because they feel it doesn't make a meaningful contribution to the world.
Second, I've watched many incredibly talented people disproportionately go work at HFT firms, and it's hard to to avoid the impression this is socially wasteful. Efficient markets are nice, but do the markets really need to be efficient on a microsecond scale? It seems like the talent sink here is unfortunate and society might be better off if some of them worked instead on, say, biotech or alternative energy sources.
I agree the the cause is obvious: finance pays more money. But it's hard to avoid thinking about solutions that reduce the profitability of HFT and don't harm market efficiency significantly, for example eliminating the subpenny rule [0]. It seems clear to me that if the current incentives result in one industry essentially monopolizing the nation's most quantitatively able workers, then those incentives might need to change.
I don't think this point was made explicit in the original post you responded to, but it lurks in the background.
Finally, I really wish you would treat other examples of market participants. Because right now you've done the easy cases, the ones that I, a total outsider, can mount a defense for. Again, it's the other ones that are interesting. And it's clear there are a ton of participants who are a) not aimed at increasing access for retail investors and b) not market makers.
[0] https://www.chrisstucchio.com/blog/2012/hft_whats_broken.htm...