It’s hardly theoretical. We know that time travel and time alterations just create new branches with identical laws of reality and physics, but whose outcomes are nevertheless different due to decisions and interference. Like the universe itself, these timelines are ever expanding and ever moving further apart.
There is some philosophical debate about whether or not parallel realities even exist, or if they are just time branches which have, for reasons yet unknown, had their universal laws tampered with or changed to create environments that no longer adhere to what we consider, Nenat tor-tal — or prime physics.
Time travel is dangerous primarily because it allows an individual to become a God, a creator and destroyer of universes.