The "what will not make it let the smoke out" will be shorter.
Almost all motors and generators are wound with what seems to be bare wire. This wire is coated with an insulating varnish to keep it from shorting turn to turn. Anything that allows that thin insulating surface to break or melt will cause problems sooner or later. Dirt, vibration, excessive heat...all the stuff found on a job site will kill a welder in a short time.
There is a word that many have never heard and that is "maintenance". If you keep it clean and do what the book says most machines will last longer than the operator.
I have had motors that were on the edge and cleaned 'm up and put 'm back and they are still running. There is no reason an old welder shouldn't last forever, especially those overbuilt things that they don't make anymore.