I suspect that it depends on what aspect(s) of prostitution they oppose; because the factors behind it also drive an increase in supply. With the supply increase in the picture, I'd be inclined to say that it's probably a bad thing.
If prostitutes had other (good) options, a decrease in demand would drive a decrease in supply, as people either left the market or never entered it in response to the poor returns available. Since that isn't happening, and people are entering the market (and, more specifically, even entering more dangerous areas of the market that experienced prostitutes historically preferred to avoid), the picture seems to be fairly bleak.
If one derives some moral satisfaction from former Johns now too poor to buy, I guess there's that; but I'd be a lot more concerned about the prostitutes who, in increasing numbers, are dividing up a smaller return, sometimes for more dangerous work, and who (based on the fact that they are still entering the market, despite the fact that doing so has become less attractive) apparently have few or lousy other options.