On a door built around 1983, the spring rarely fails alone. The cable drums on these older setups wear a groove from decades of the same cable path, and once that groove deepens, a new spring won't seat the cable correctly - you end up with a cable that jumps the drum under tension, which is a different repair than the spring itself.
Cold is the other factor. This part of Kansas sits in a moderate-cold climate band, and a spring that's already fatigued from age will often pick a freezing morning to let go, because the metal's less forgiving when it's cold and the door's been sitting closed all night. We see more spring calls cluster in the cold months here than any other time of year.
The honest limitation: if the drums, bearing plates, or shaft are worn enough that a new spring would just transfer the same stress to the next weak part, we'll tell you that replacing just the spring isn't worth doing - sometimes the math points to replacing the whole torsion assembly instead, and we'd rather say that than sell a repair that fails again in six months.