APAD: If it ain't broke, don't fix it.

As a proverb, it must have circulated for a long while.

The Greek has the word "iatrogenic," of harm done by the healer. One principle of doctors is to try not to harm. The head of a modern drug company famously said that they should make money by selling not to cure the sick but to the mass of healthy people.

Contemporarily, it is a familiar adage among software programmers, i.e., if the system works, don't try to "improve" things by making changes.

Here, "ain't" is a contraction of "is not."