I'm not saying you're wrong. By using your example, this documentary is all about the traveling around by foot reading century-old books, not about if you'll get to Mars. Why does that have to matter so much? It's an interesting documentary about an interesting person, devoting his life to something that makes no sense in most people's eyes. That is the entire point.
Even my point about the alchemists coming up with working non-transmutation processes is not because I try to state "maybe it's possible", because I really don't. However, we're not living in a world where you have to be right for your choices to take you somewhere interesting. Think microwave oven, the post-it note, Västerbottensost, or anything else that was originally created partially by accident. I mean, hypothetically - devote your life to studying alchemy and you might end up having knowledge enough to teach history at an university. That's a win to me. As I see it, we're fragile creatures lost in time and space, we can only see a few steps ahead of our decisions and can never be sure where life will take us. Our wrong decisions can take us to the right places, and things that are right and carefully planned can go horribly wrong. If you want to devote your life to something impossible, why not go for it and enjoy the ride?
I'll personally be happier if I die knowing that I did something I enjoyed, than if I die knowing I managed to stay objective and right through my entire life.