4 ideas
When he returns to 2011, he finds a ruined present of nuclear exchange, earthquakes from unravelling time, and political collapse. He goes back through the portal to reset everything and gives up the life he had built with Sadie.
The past acts as a force that pushes back against attempts to alter it, and the harder it pushes the more a change matters. Cars break down, illnesses strike and accidents pile up in proportion to how large the intervention is. Small changes pass with little friction, while stopping a presidential assassination draws violent resistance.
Removing a single terrible event from history does not produce the better world its absence seems to promise, because the event is woven into countless dependent outcomes. Kennedy surviving leads through a chain of consequences to a worse world, not a better one. The lesson is that tragedies with a clear cause can look easy to undo, yet undoing them changes everything built on them in ways no one can predict.
Events in the past echo one another: names, places and patterns of violence recur across unrelated lives, as when Jake's Derry and Dallas experiences mirror each other. Seen this way, repeated coincidences are read as the structure of the system showing through, not as random noise. That repetition then warns you when you are approaching a point where things may go wrong.