You make a good point. All sorts of species and genera died out with the Cretaceous-Tertiary extinction. Most people only focus on the larger animals, while ignoring the plant kingdom. But there was another natural kingdom to consider - the microfauna. I spent quite a bit of time in my career working with micropaloentological data. These data often provide a better, more complete record of climatological change than do terrestrial fossils. I do not believe the micropaleo records supports a "sudden" extinction.
What is your take on this then? A series of cumulative disasters?
You are wrong. See T. Rex and the Crater of Doom. The clearest evidence of an abrupt extinction is the abrupt and total change in the foramintifera.