Isn't it strange how everything is connected? One event leads to another leads to another leads to another, until something completely different happens that never could've taken place without the first thing in the chain of events, even if it seems completely unrelated.
For example, Morgan could've gotten away with poisoning the entire earth if not for the fact that she offered Elam fruit from her tree to bribe him thousands of years ago. The poison didn't work because Shiloh had the plant from the sixth circle, which she only had because Sapphira planted it there so long ago, which she couldn't have done without the seed from the fruit from the tree she only had because she planted the flower Elam gave her from the tree before that one.
If not for Palin and Devin achieving whatever form of immortality they had, Palin wouldn't have met whoever his wife was so many years later, and Marilyn wouldn't have existed, and Billy wouldn't have existed, and so much of the story wouldn't have happened.
If Makaidos hadn't been murdered when Devin first started his dragon slaying, then Makaidos wouldn't have gone to Dragons' Rest and been brought back, and Gabriel and Ashley wouldn't have been born. If Gabriel didn't exist, there never would've been that invisible guardian angel in the lives of all those he protected. Without Ashley, the lab under the mountain wouldn't have existed, and there would have been no translumination back from a light form; all of the people trapped in the candlestone couldn't have been rescued.
A lot of times, it only takes one choice to change a whole chain of events that can go on for years. To go back in time and change anything would result in an extremely different state of the world later on, same as any small choice made today can influence the state of the world in many years to come.