The set of problems that any particular design will answer (it's a general thing, not just software) will change over time as economies change. That's why you p

The set of problems that any particular design will answer (it's a general thing, not just software) will change over time as economies change. That's why you probably don't own a horse, and certainly don't ride to work on one. Software may be 100% stable and yet will become gradually less accurate over time, doing work no-one needs and yet which places a cost on the project. You cannot prevent this. The right thing to do is to learn to abandon old solutions when they're no longer economical, and to move rapidly to build new ones that fit the same APIs, yet know more about the modern world. Hence your car seat is approximately the same API as a horse saddle, built on different technology.