> They lived at a time when there were surely more asteroids hitting the Earth than now (the system was younger, so there was more debris flying around), so they surely had more warning than we do: craters, smaller impacts, etc.

This is a tangential point, but they weren't living in an appreciably younger solar system. 65 million / 4.5 billion is ~1.5%. So, the solar system (and the Earth) was 98.5% of its current age.