Good point. We already have the K-T layer smoking gun so the asteroid impact is without a doubt.
What may be in doubt is if there is any other contributor and how long some of the dinosaurs survived. It could be that the asteroid had some other lasting effect on the earth, even causing a minor shift in orbit or tilt that helped do away with the tougher dinosaurs.
Is there any other contributor?
The answer is yes, possibly. An Indian scientist, Chaterjee (sp?), is positing a much larger impact of roughly the same age called the Shiva Crater. This crater is supposed to be 400 by 600 kilometers in diameter as opposed to the "mere" 180 kilometers of the Yucatan crater. It ajoins the Deccan trappes (volcanic effusion some 9,000 ft thick and more than 300 miles in diameter which is also of roughly the same age.
Whether the Deccan was the result of a Shiva impact or not, it also would have done terrible things to earths atmosphere for quite a long time. Poor dinosaurs!!