I think time travel is possible, but you can only change things from the perspective of the person who travelled back. The original timeline just keeps on going.
Person (a) travels back in time to kill Hitler and succeeds. That new timeline exists now without Hitler, but in the original timeline is already written in stone. There is no changing it. The moment that person travels back, they've created a new timeline. It's both impossible for them to change the original timeline, and impossible to ever return to the original timeline because if they go back to the future they'll only be travelling along the new timeline.
100% agreed. Such a fun show.