Yes, but some groups of dinosaurs were declining before the asteroid hit, due to climate change, competition, sea level fluctuation, and volcanic activity. Some groups of mammals were getting more advanced and diversifying, though they were still mostly small, nocturnal insectivores.
Primitive cats, dogs, primates, and others evolved before the asteroid hit. Dinos were still King of the Hill, but their world was becoming more competitive. Competition and environmental change spur evolution. It was time for them to adapt or perish. Fish, or cut bait. Shit, or get off the pot.
I don’t think the non-avian dinosaurs would have relinquished their crown without a fight. They had to evolve or die. They had to survive or go extinct.
Some obvious pathways for them to evolve and continue their dominance would be to gain more intelligence, become more social, and perhaps even become fully warm-blooded.
So, I think it’s fair to say that mammals were becoming more competitive with dinos, and that probably would have continued if the bolide collision didn’t occur. Both dinosaurs and mammals would have continued to evolve.
One thing is certain, our world today would have been much different if that asteroid missed hitting our planet 66 million years ago. For one thing, I don’t believe we humans would have evolved into existence at all. And, if we didn’t exist, we wouldn’t be facing the Holocene extinction today. Frankly, our biosphere would have been better off without us.
So, on the one hand, I’m happy the asteroid hit our planet. Otherwise, we wouldn’t be here to experience the hedonistic qualia you and I experience from birth to death.
But, on the other hand, I feel guilty that my species really fucked things up for us and the rest of the species on Earth.
Maybe the next dominant species will do a better job. It can’t do much worse.