I'm not an astronomer, but my understanding is that when it comes to impact events, the probability of a planet-killer probably is going down over time: after an impact event, the impactor isn't around to pose a threat anymore, and there are only so many large objects with Earth-crossing orbits. At this point we're far enough out on a long enough tail that the probability density isn't changing much over time, but that's not true over the timescales being graphed; if we're dating lunar rocks accurately, extinction-scale impact events last peaked around 3.9 Gya, and were rare after 3 Gya. I'd be surprised if that trend hasn't to some extent continued.
More generally, if you want a good estimate of the near-term probability of impact events, you probably want to survey one or several of the other bodies in the solar system. They have an impact record relatively untainted by anthropic bias, and also have the advantage of being a lot easier to read, as most of them lack the plate tectonics that wipe out a lot of older geology on Earth. That said, though, there are extinction events that wouldn't be affected by the later evolution of the solar system: nearby supernovae, for example, or gamma-ray bursts.
More generally, if you want a good estimate of the near-term probability of impact events, you probably want to survey one or several of the other bodies in the solar system. They have an impact record relatively untainted by anthropic bias, and also have the advantage of being a lot easier to read, lacking the plate tectonics that tend to wipe out a lot of older geology on Earth.
And this has been done. We have good records of impact levels on the Moon, Mars and Jupiter (although Jupiter is a little weird). It doesn't look like there's heavy anthropic bias there.
From a paper by Milan M. Ćirković, Anders Sandberg, and Nick Bostrom:
There cannot have been a large disaster on Earth in the last millennia, or we wouldn't be around to see it. There can't have been a very large disaster on Earth in the last ten thousand years, or we wouldn't be around to see it. There can't have been a huge disaster on Earth in the last million years, or we wouldn't be around to see it. There can't have been a planet-destroying disaster on Earth... ever.
Thus the fact that we exist precludes us seeing certain types of disasters in the historical record; as we get closer and closer to the present day, the magnitude of the disasters we can see goes down. These missing disasters form the "anthropic shadow", somewhat visible in the top right of this diagram:
Hence even though it looks like the risk is going down (the magnitude is diminishing as we approach the present), we can't rely on this being true: it could be a purely anthropic effect.