[xianshou]

Re: "The simplest way to estimate the future is to guess that it will be similar to the past."

When we say that the future will be similar to the past, for code, we really mean that the probability of certain events occurring in the future will be similar to their prior probability of occurring in the past.

In my hypothetical example of testing an invariant that is unlikely to fail but damaging if it does, it might be valuable to keep that test around for five years even if it never fails. Imagine that the expected frequency of failure was initially <once per ten years>, and that the test hasn't failed after five years. If the expected frequency of failure, cost of failure, and gain from fixing a failure remain the same, we should keep the test even if it's never failed: the expected benefit is constant.

Not to say that we should test for every possible bug, but if something is important enough in the first place to test for it, and that doesn't change (as calculated by expected benefit minus expected cost of maintenance), we should keep the test whether or not it changes our behavior.

Thus, if we could estimate probabilities correctly, we really would know when it's OK to nuke a test.