Here is the honest limitation: on a door that is original to a house built before 1970, we sometimes cannot just swap one part and call it fixed. The track width, the hinge spacing, even the panel gauge on some of that older stock does not match anything made today, so a repair on one section can mean the rest of the door starts failing in six months because it is now carrying more stress than the worn parts were built for.
When that is the case we say so instead of billing you for a patch that will not hold. If the springs, the bottom panel, and the opener are all near the end of their life at the same time, replacing the whole door usually costs less over three years than three separate repair calls. We will not talk you into a new door just because the house is old - plenty of 1960s and '70s doors around here are still sound and only need a spring or a cable. But if the math does not work, we tell you before we start, not after we have charged you for the first fix.