In addition to causes others mentioned I'd like to add bit rot. This will however most likely not affect your 70s or early 80s watch as those back then was made with mask rom (hard wired, ROM data defined in the etch mask when making the IC and cannot be changed). Many later watches and certainly today watches store their "computer's" data in an EPROM which is a flash like device. Depending on mask size and technique when made it'll be a finite time before the crucial data to operate watch evaporates. Basically the charges are static charges stored in cells with an extremely well insulation. However no insulation is perfect and despite of earths atmosphere it's exposed to high energy radiation. Like the static charge that hold a balloon to the ceiling it too will evaporate with time.
I can't give an exact time of when the EPROM method was becoming common instead of the earlier mask ROM but sometime in the mid to late 80s would be a good guess (and was dependent of the thing it was to be used in). Beside earlier analogue might contain an IC but it's purely a simple collection of deviding logic as regulation is purely analog. While there perhaps is a theoretical wear on the IC's internal transistors etc. it's very small and if not exposed to any of the mentioned hazards it'll keep on working till the atoms that once made up the earth is swallowed by the black hole in the center of the Milkyway.