[Archelaos]

A single event like the Shoemaker-Levy 9 impact does not tell us much about frequencies on its own. Besides, the original width of Shoemaker-Levy 9 was estimated to have been between 1.5 to 2 km.[1] The Chicxulub meteorite was estimated to have been between 10 and 80 km wide.[2] And it happened around 65 million years ago. So frequencies then and frequencies now might be quite different. That meteorite impacts happen from time to time on earth was nothing controversial. The question was, whether there had been a (very rare) hugh impact, large enough at the time of the Cretaceous–Paleogene extinction event and whether this was the only cause for all those extinctions. This is a very complex scenario and several of its details are still debated today.[3]

The most important evidence that an impact was the main cause of the extinction event was provided by the discovery of the Chicxulub crater in 1990/91. However, the investigation of this and other evidence is still ongoing. Contrary to popular belief, scientific debates of this magnitude are not resolved by a single ingenious theory or observation. It is the hard work of many, many people over years and decades that gradually changes and refines the web of belief of a scientific community.

[1] https://solarsystem.nasa.gov/asteroids-comets-and-meteors/co...

[2] https://arxiv.org/abs/1403.6391

[3] Wikipedia offers summaries of some alternative hypotheses, showing how complex the arguments are: https://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_e...