Mechanical watch - whether it is Manual winding or automatic, all its parts are metal and now with some silicon and other new material
There is no maintenance free mechanical watch created so far to my knowledge !
What pushing a watch to run is transform of energy, whether it's from hand wind human power or wrist movement causing the auto winding rotor swinging to wind up the barrel spring... Then it release the barrel spring release it's tension bit by bit due to escapement mechanism to push a watch to run as well as other mechanism to work. Wheel, gear, clutch.... All have surface touching/intact area, a good and lasting watch will have a polished/smooth/hard at this area, less friction less maintenance and last longer!
Ruby, Ceramic, dlc, diamond, silicon... All been introduced to improve hardness, reducing the usage of oil. Oil has been improve in watch making, but still widely used, as long as oil is used, it will dry and oxidised. Hence left with dirty and rough surface, area touching it will be worn out day by day. Hence your watch will be broken down once the worn part too serious
That is why most watch been advised to go back for servicing ard 5 yrs. that's the time the oil dry ...
It's still depend on how often the watch run, how well it is sealed off from moist....
Having said that, opening a watch for servicing also, will damage your watch to certain level, ie - each time you unscrew, the thread is worn! Watch maker are human as well, mistake will be made, big or small.
Many watches so call no problem after 15 yrs never service and still running well.... It's actually problematic to certain extend to the eyes under the loupe!!!