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.