No one wants four-trades-a-day settlement to save 0.00001% or whatever in trading fees.
That's true, we don't want it to do that, we want it to kill these parasitic entities. Much like one doesn't swat a mosquito because one will truly miss the amount of blood she's taking.
No one? Mutual funds have managed to attract $33 trillion trading once a day. The demand for millisecond-level trading is almost entirely from a very small group of firms profiting from it.
And they are steadily losing new investment dollars to ETFs, which trade interday. I don't think interday trading is why ETFs are more attractive to all or most investors, but a 0.000001% (or whatever) cost advantage just falls below the noise floor. It isn't worth any other tradeoff.
Then the real trading will just move to hyper liquid or another platform that allows trading in real time.