I think in the end they've tried to be over smart about it and suggest the roles would be reversed.
Couldn't they have used another dinosaur, perhaps a stegosaurus or triceratops, having a human kid just took the biscuit I think.
No, bc spot and Arlo were so different yet had things in common was a big deal. If it was another animal or another dinosaur, Arlo wouldn't have had as much fear as he would've with a human that he was completely unfamiliar with. It made their connection more powerful and meaningful.
shareThey thought that having a human child would access the dream that some kids have of being able to play with dinosaurs.
"Baba Booey! Baba Booey!"
The opening shot is the answer.
Had the asteroid have hit the earth it would have wiped out the dinosaurs (like the extinction level event millions of years ago).
In this alternative universe dinosaurs have evolved to create habitats and survive like humans did in the old west of America.
Humans haven't evolved as they would because dinosaurs have remained the dominant animals on the planet, humans are feral creatures still in evolutionary development.