How it all started
So What is this Theory About
In a brief summary: it's our doomsday-story. But is not quite that simple because it has magic, animals, monsters, emotions, global warming, artificial intelligence (AI) and much more.
For starters we need to back up all the way to the 14th/15th Century, where in medieval Scotland with "Brave" we are enlightening on why in stories like "Finding Nemo", "Ratatouille" and "Bug's Life" the animals are featured with enhanced human characteristics, being able to speak to other species and interact in a type of society. And the answer is MAGIC!
If you don't remember the plot of "Brave" the most important point is that Merida's story guide us thru her mother being magically turned into a bear and yet remaining with some human aspects, specially her consciousness. But there is a catch that can quite explain why in the future of this timeline there are still animals - like Andy's dog in Toy Story. Negroni says:
(...) animals in Brave gradually regress back into an animal state, disproving the idea that this is the source of animals acting like humans. My rebuttal is simple. They regress because the magic wears off. Over time, their evolving intelligence grows naturally
The rise of the machines?
Energy Crises and the Beginning of the End
What about those animals?
The Uprise
We know that the machines will win the war, too, because after this war, there are almost no animals left on Earth. Who's left?
Energy Crises 2: The Return
After WALL-E: humans, animals and monsters
Monsters are most likely a result of radioactive mutation and probably (although I prefer NOT to think about this) inter-species breeding.
Since the events of Monster's Inc are set thousands of years in the future - probably because the reset the calendar years after the return to earth - it is possible that a monster society, with different distinctive characteristics is a result of this process.
Energy Crises 3: FEAR THE MONSTERS
The plot of Monsters Inc. lands us the final piece of the theory and -- prepare to have you mind blown -- its beginning too!
The story begin with an energy crisis that is - yet again - upon the citizens of Monstropolis and if you remember well (because that movie was released 15 years ago) they harvested their energy from children's screams so why is there any energy crises?
Well, for one they need to keep traveling either through time and/or dimension to find humans that - yet again - are link to production of energy and the fact that this timelines are approaching, the children are no longer scared of the monsters. Even worst, when BOO comes to the monster future, she becomes aware of this pivotal elements that created the future of mankind, messing with the whole space-time continuum. The first problem is solved by laughter but the second...
BOO: the beginning and the end
After 2013: How Inside Out and Good Dinosaur fit in
In 2016, when "The Good Dinosaur" was release yet again people focused on how to make it fit in a story that began in Scotland centuries lated. Well here is their solution:
Millions of years passed since the asteroid that was supposed to wipe out the dinos never smashed against earth, therefore the dinosaurs where able to evolve and live in the same period as humans did.
Since dinos are so much bigger then humans they kind off limited the human development, since they were the "top dogs". But as the movie progress you are introduced to the dinos mean of extinction: the weather. That goes with the theory that a reduced population of dinos facilitated the group of humans that actually knew how to work with tools could become the dominating species.
Bottom line: the place of the movie in the theory is to set up the presence of humans in the timeline.