I’m sure you’re right, as it relates to brain development. But, when sudden climate changes occurs, having the ability to relocate, quickly…. And knowing where to go, was a big advantage. I doubt the large herbivores were highly mobile. Food sources dried up quickly.
People have pointed out that the survivors of 65mya were mostly small creatures who could hide or like crocs who hid under mud banks and only ate occasionally. Another suggestion was that the blast through the atmosphere destroyed most of the protective ozone, leaving large bare skinned dinosaurs vulnerable to skin cancer. On the other hand birds were covered with feathers which gave them protection and in addition to being able to hide, the small mammals also had hair on their bodies.