Assuming 0.1c travel, the galaxy can be covered in a million years already, which is way less than the lifetime of many species here on earth. But if you extend

Assuming 0.1c travel, the galaxy can be covered in a million years already, which is way less than the lifetime of many species here on earth. But if you extend your argument to intergalactic travel, then you definitely have to start talking about billions of years. I generally think that this argument is the best one that FTL travel is impossible, because it would have increased the likelihood of non-human intelligent life close but outside of earth dramatically. Outside of course physics of the last 160 years being built on the fact that C is the fastest speed there is.