To be able to say that something is impossible, you need absolute evidence of the fact otherwise someone with more vision will make a fool of you in the future. Saying for example that FTL is impossible just because nothing in nature does it is a fallacy. There are plenty of things humans do that are 'unnatural', how many other creatures on this planet deliberately take drugs, cook their food, kill other animals for any other reason than food or pack dominance. The whole history of humankind sees ingenious people going against the accepted wisdom and coming up with new ideas. It would be better to say something's "impossible with our current technology" than to say "it can't be done".