But your 286 is already exceptional, how many other computers of that vintage are still functional today? This is analogous to particle decay in a radioactive material, you can't predict accurately when any given atom of uranium will decay, but you can describe the rate of decay by the time at which close to half the particles in the sample will have decayed. The half-life of the typical consumer grade PC is ten years or so, from what I've seen (this is an estimate, but this would be a good research project for someone), but just because a model of a given computer still works after 20 years, doesn't mean it will work in another five.