everything we want is as possible and makes as much sense as wanting to hear the sound of blue or taste the flavor of a prime number
We know it isn't because most of the time we get what we want. You want chocolate, so you go and buy some and then eat it, and the yummy chocolatey taste you experience is proof that it wasn't that futile after all for you to want chocolate.
The feeling of reward we get when we satisfy some of our terminal values is what makes us think that they aren't based on a misunderstanding of reality. So it's probably a pretty good bet to keep wanting at least the things that have led to rewards in the past, even if we aren't as sure about the rest of them, like going back in time.
Let's say Bob's terminal value is to travel back in time and ride a dinosaur.
It is instrumentally rational for Bob to study physics so he can learn how to build a time machine. As he learns more physics, Bob realizes that his terminal value is not only utterly impossible but meaningless. By definition, someone in Bob's past riding a dinosaur is not a future evolution of the present Bob.
There are a number of ways to create the subjective experience of having gone into the past and ridden a dinosaur. But to Bob, it's not the same because he wanted both the subjective experience and the knowledge that it corresponded to objective fact. Without the latter, he might as well have just watched a movie or played a video game.
So if we took the original, innocent-of-physics Bob and somehow calculated his coherent extrapolated volition, we would end up with a Bob who has given up on time travel. The original Bob would not want to be this Bob.
But, how do we know that _anything_ we value won't similarly dissolve under sufficiently thorough deconstruction? Let's suppose for a minute that all "human values" are dangling units; that everything we want is as possible and makes as much sense as wanting to hear the sound of blue or taste the flavor of a prime number. What is the rational course of action in such a situation?
PS: If your response resembles "keep attempting to XXX anyway", please explain what privileges XXX over any number of other alternatives other than your current preference. Are you using some kind of pre-commitment strategy to a subset of your current goals? Do you now wish you had used the same strategy to precommit to goals you had when you were a toddler?