An asteroid may kill a lot of people, but it will not cause global extinction. No asteroid strike has ever completely wiped out life on earth.
Isn't that argument a bit like "I plan to live forever, so far so good"? After all, if it did wipe out all life well then we'd be dead so obviously it hasn't happened yet. Some large extinction event seem to happen once every 50-100 million years, what does a once in a billion year event look like? Ceres, the biggest object in the asteroid belt is about a million times bigger (10^20 kg vs 10^14 kg) than the dino killer. That one isn't going anywhere, but there's clearly quite a few potential total extinction candidates if they came to intersect with Earth's orbit.