These sort of calculations are nothing but examples of how a bad model gives bad results. In this case, the model is bad because it's underspecified. Saying "I

These sort of calculations are nothing but examples of how a bad model gives bad results. In this case, the model is bad because it's underspecified. Saying "I have a bad model but there's some unknown Great Factor which would make it a good model" isn't very convincing. There are myriad factors: the biological challenges of space travel constitute a long list in itself; the economics and politics involved (ROI on any useful timeframe for interstellar travel is zero); the engineering challenges (the vast majority of interstellar travel attempts will result in the death of all crew somewhere in interstellar space); the fact that over million-year timescales, species evolve and change - humans are only around 30% of a million years old.