The ozone layer isn't going to do anything to stop a comet or asteroid impact whether the ozone layer has holes in it or not. The ozone blocks UV radiation, it has no effect on protecting the planet from impact events. The earth gains around 15,000 tons of material every year from various size objects running into the planet. A strike like the one in Russia comes along about every 30 or 40 years. There is a constant bombardment of stuff that is smaller. Since the planet is covered with about 70 percent water most of these events happen where there is no no to see them and the ocean swallows anything that makes it to the surface. The little stuff burns up at high altitude and isn't noticeable from the ground.
Technologically we are are at the cusp of being able to prevent or deflect a major impact event. One of the problems is that there are probably a few million rocks floating around the solar system like the one that hit Russia. While these aren't going to cause damage on a planetary scale a direct hit on a major metropolitan area could be devastating. I was really surprised there were no fatalities with the Russian event given the number of injuries.
Major impacts like the one at K-T boundary happen only every several million years. If one of these was heading toward us we would be pretty well screwed unless we has a couple of decades advance. However if we had a long period comet out of the Oort cloud and we had only a few months warning, then go max out the credit cards, get layed a lot, and tell your boss to bite your ass.It is highly unlikely that humanity would survive an event like the K-T one. And if we did civilization would be starting over. The lucky ones would be the ones that died right away.