Originally Posted by Byron
Henry will likely give you a much more experienced response. Usually "running way too long without a service" will cause the watch to stop after a period where the watch will gain a lot by the lost an optimum amplitude of the balance. It could also block for diverse reasons : a pivot broken, a pallet detached for the escapment, the spiral dirty or displaced.
The best diagnosis is given by an analysis of the watch run is different positions . An acceptable isochronism should be recorded. If not, a service should be done and each parts inspected for acceptability.
For about 15 restoration project I restarted successfully some real wrecks without changing more that a few parts (a main spring, a wheel with worn pivot, a broken stem...). Here that ETA 2472 auto from the 60's as an example:
I would say that, excepted in special cases, the replacement parts necessary are usually a marginal cost in the service of a watch in reasonable condition.