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.