I didn't think that a digression about the tax optimization opportunities of in-kind securities swaps would be enlightening to a general audience.

As for your second point, I guess neither of us works at Citadel so we're both guessing, but if you agree that there is a market for order flow and that the most charitable interpretation of why that would be the case is because of the attached optionality for internal netting, then you're sort of making the assertion that all PFOF is equally valuable, which would be a hell of a coincidence.

As a sort of side note, I regard the cherry-pick the parent with ">" prefixes and go after snippets of what they posted as basically the lowest form of discourse on HN in spite of how popular it is, and I would encourage anyone to try to reply to someone's entire comment rather than just trying to find the weld points and lean on those spots. It's pretty weak.

As I've mentioned elsewhere in the thread, it's notoriously difficult to get citations on this stuff so take with a grain of salt, but I've heard that in 2019 the "HFT" industry (defined some way) had cumulative annual profits in the US of somewhere between 2-4 billion dollars. That's a long holiday weekend for Google or FB. I've also heard that this (inflation-adjusted etc. etc.) this is down sharply from ten years before, when spreads were wide and undocumented order types were winked at.

To wildly oversimplify, market makers will tend to drive the spread down to the tick size, and arbitrageurs will tend to put themselves out of a job.

For the industry as a whole to be growing either in distinct actors or cumulative top-line, the number of markets and instruments and general financial activity has to be growing faster than the big dogs are eating each other. This is my (semi-informed) guess.