It did become extinct eventually, but not through asteroid impacts. Probably because it was unable to retract its neck and succumbed to predators.
And it is also extremely unlikely that we are hit by anything even that big. We have found all the 10 km diameter asteroids between Jupiter and the Sun already. Found 90% of the 1 km ones also. The 10 km ones happen only every 100 million years. And given that we have found all except the ones currently way out further away than Jupiter, that makes it 99.99999% certain we won t be hit by one of those this century. However, humans can live on many things, not just on land, also in the sea and along the sea shore, in lakes, underground, and in the air. Hard to think of many animals as adaptable as humans with the most primitive technology. Fruit, insects, nuts, grains, roots, shellfish, fish, animals, including rodents and other small animals, birds, seaweed, etc. If you lived exclusively on some of those you might not live very long but