There seems to be more to it than just that, or so the argument goes. The example of people wearing prosthetics to warn of damaging situations seemed relevant: they would disable the electric shock, do what they wanted, then re-enable it.

For pain to carry value it must represent the ground truth and can't be easily ignorable. That makes it carry a very large cost to override which is important since as the examples demonstrate the long-term consequences of an inability to feel pain can be quite severe.

I suppose that ties in to what you said: humans are way too bad a long-term thinking to properly evaluate consequences so we would end up burning ourselves or hacking off a limb to achieve a short-term reward. Or at least enough of us would be that our species would never have made it this far.