Are you saying that "reward = easier game"? So that the standard "game level" model of increasing challenge over time should be inverted?
I don't follow your smashed car analogy at all; they got XP and treasure from their previous encounter, their characters are more powerful and influential, and whatever story line changes they made to the world in the process of it remain (towns stay saved, princesses remain rescued, evil overlords remain overthrown, etc.).
Now, true, there will always be more adventures; but that doesn't really hold up to your "smashed car" analogy, its more like finishing one car and then deciding to start work on another car because you enjoy restoring cars.
Edit: Also, reading your post again, something occurred to me. Are you under the impression that I don't give the players downtime between adventures?