I think reasonable caution by space agencies is wise but it also could have already happened a billion years ago. If we want to survive as a species or lineage of species beyond the Sun enveloping the Earth we will also need to deliberately establish viable life on other planets and even other solar systems at some point, previous historical records of ancestral life or present planetary sterility be damned. Life seems too rare in the universe for it to go down with the ship, we should make an effort to duplicate this experiment even if humanity doesn't make it.