Ok, but it's not the same half million on any given day. A huge number of those people got picked up the night before and will be out (on bail or otherwise) by the end of the business day. Incarcerating people is expensive. Even if getting them out doesn't involve the state getting an interest free loan the state still has an incentive to get them out.

In functional jurisdictions the turnover is high. I know that there's a litany of exceptions for the authoritarian (and therefore more inclined to lock people up for no good reason) and well moneyed (and have the money to do it) jurisdictions in the Boston-DC corridor (and please nobody construe this as me giving the Southeast or west coast a pass, they suck too but do it differently) but the underlying problem there is societal. If you reform bail the well moneyed authoritarian jurisdictions are still well moneyed and still authoritarians and will simply spend that money using the system to screw people some other way.