It's just one of those odd cosmological questions with limited practical value that I tend to occupy myself with, especially when I'm supposed to be doing actual work.
And by "wear out" I mean why aren't they subject to the 2nd Law of Thermodynamics?
Atoms are often involved in entropy, but they themselves seem to be mere "carriers" of it since they remain unchanged throughout all such interactions (with the exception of rendering them into plasma, reactions with anti-matter, and nuclear fission, which may be clues...).
Atoms created in stars and supernovae billions of years ago are still the same as they ever were (as far as we can tell anyway). They may take part in countless chemical (and possibly some nuclear) reactions over that time, combining into various molecules and returning to their elemental form and then reacting again (or being rendered into heavier elements within stars, but not lighter elements, oddly enough). They also might just "sit there" stuck inside a rock floating through space...forever? I almost feel sorry for those atoms that never get a chance to know the joys of exothermicity, but that would be weird.
Anyway, when they do get to react with other atoms, the reactions are sometimes endothermic and sometimes exothermic (and the 2nd Law is dutifully upheld), but the atoms themselves endure through it all, making them seem a whole lot like little "perpetual motion" machines, which are supposedly impossible at our scale of physics but are apparently a-ok (and common) at the quantum level.
So... what's up with that?