What Happens If You Don't Service an Automatic Watch?  Maybe nothing.  It might never need servicing in your lifetime.  An automatic watch movement could run for decades and never need servicing. It does happen. Of course if the timing gets out of spec for the movement model it will need servicing.  It could just need an adjustment or possibly a complete overhaul.  So unless your automatic watch movement is giving some type of problem no service is needed.

There was  time when the lubricants used in an automatic watch movement would "pool" of you let a watch sit too long without wearing it and cause issues.  These days modern watch lubricants do not really do that so it is not as much of an issue as it used to be. It is best practice though not to let your watch sit for months or years without running it.

Your movement becoming magnetized could cause the timing to be off.  If you are lucky the watchmaker can just  demagnetize it and fix the issue.

This brings us to another topic.  Watchmakers. Think of a watchmaker as a mechanic for your watch.  That being said a second opinion is always good.  In our experience most watch makers want to sell "full service" overhauls on virtually every job.  That means a bill typically for $500-$800.  Usually this is not necessary.  Try to find a reputable watchmaker that you trust and is honest.  Many watch movement issues can be fixed rather simply without much time by a watchmaker that knows what he is doing and not trying to sell full service overhauls to everyone that comes through the door.  Keep in mind that if your watch has a common, mass produced automatic movement (Miyota, Seiko, ETA, Sellita) that often the movement can be totally replaced for less than having a  watchmaker do a total "overhaul".