"My point is that ability to survive space travel will never be a trait that is selected for by evolution, because the advantage of the trait disappears in the countless generations that intervene between the rare times that space travel becomes useful to an organism."

Children are rarely useful to an organism.

Spreading life to other planets, seems like the ultimate survival trait. Any life that could spread life to other planets would have to be considered one of the most successful.

Most life on any planet won't survive a large impactor.
So if one is looking at time periods of billions of years, those life forms that can survive a large impactors, a close proximity to super nova, or the death of a Sun would have the best survival tract compared to any other life forms which are unable to do so- and organisms which can do this probably dominate this Universe or at least in time will.

So. If Earth microorganisms can manage to get humans off this planet, they will be one of the most successful. Otherwise, humans will not have been of much use for the organism.