When a measure becomes a target, it ceases to be a good measure.
cf Campbell's Law: “The more any quantitative social indicator is used for social decision-making, the more subject it will be to corruption pressures and the more apt it will be to distort and corrupt the social processes it is intended to monitor.”
Goodhart's Law Isn't as Useful as You Might Think: Yes, this is a real phenomenon, but it can be mitigated pretty effectively (with deliberate, intense effort!)