Good Question:
Here are the parts that were just replaced in an overhaul of one of mine:
Let's see, I can make out the Stem Tube, Pallet Fork and Jewels, Winding Pinion, Center Wheel, Escape Wheel, Stem Detent and Stem, A bridge...and some other stuff..
None of it looks worn out to me........
This whole mess of stuff doesn't take up much more space than a shirt button..