Agree and disagree. The problem is time, but not travel time. It is time spent off-planet. Our biology is 100s of millions of years old (at least).

We are perfectly adapted to living on spaceship earth. As we have seen over 70 years of space travel, even short times on other spaceships leads to a very quick degradation of our biology. Not to mention, we have to bring a not insignificant portion of "earth" with us when we travel. Water, food, atmosphere and some type of gravity. Extended travel means we need to create an artificial biosphere to sustain us. If after 50 years on the way to some star we discover we missed a crucial enzyme while "packing" for Proxima Centauri, the entire crew could die before they can get back to earth.

Fortunately, we have a billion years to figure this out. Personally, I think we'll be lucky to finish this century with a fuctioning civilization. One small step for man...