I do agree that there are things you might think you want that don't really make sense given that in a few hundred years you're likely to be long dead and your influence on the world is likely to be lost in the noise.
But that's a long way from saying -- as bokov seems to be -- that this invalidates "everything on which we base our long-term plans".
I wouldn't spend the next hour reading a scientific book if I knew that at the end my brain would be reset to its prior state. But I will happily spend time reading a scientific book if, e.g., it will make my life more interesting for the next few years, or lead to higher income which I can use to retire earlier, buy nicer things, or give to charity, even if all those benefits take place only over (say) the next 20 years.
Perhaps I'm unusual, or perhaps I'm fooling myself, but it doesn't seem to me as if my long-term plans, or anyone else's, are predicated on living for ever or having influence that lasts for hundreds of years.
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?