Why should we care about whether someone speaks or not?
uh... because it's an action that was not previously being done that (hypothetically) is now being done.
Like, sure you're claiming that someone is far more likely to act based off of torture, most likely based off your internal model of how you would act, but such a model is by no means representative of how other people or even yourself would act.
People being tortured at all is fairly obvious evidence that people are more likely to act based on it, because if the interrogatee were already talking there would be no goddamn point in torturing them for something that's not even guaranteed to be the truth. And while the latter is certain to have happened at some point, it seems flatly stupid to think it's the standard.
This isn't even counting the fact that you don't make any justification for why you consider the incredibly vague term "torture" equivalent to "all the riches in the world" and while it may seem like only a semantics squabble, and that you actually just mean "any really pleasurable event" it really isn't. There's a big difference between, let's say "all the riches in the world" or "the ability to experience the maximal amount of pleasure possible for X amount of time" or "harem of 50 raven haired Brazilian belly dancers (if you were so inclined)" simply because it may be that torture simply cannot be negated by anything we can do now or that it's harder to think of what specific things you can do with X amount of money rather than picking for someone.
The hypothetical wasn't "we will STOP torturing you and give you this really cool thing." It was simply a choice before either begins. And yes, "all the riches in the world" is very directly a substitute phrase for "super great thing you want" because I was being lazy. You could let our hypothetical prisoner ****ing pick whatever they'd want and it wouldn't change my point that pain seems a more motivating method to convince someone to play ball. Of course my hypothetical prisoner is a man of conviction rather than a common criminal, who probably wouldn't need much pain or pleasure to fess up, but the stronger motivational force would be proven in the extremes anyway wouldn't it.
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I ****ing love to cuddle.
I'm being oppressed!