I agree they are basically saying the same thing but there is a subtle difference. Goodhart is observing a fact about measures, Campbell is offering an explanat

I agree they are basically saying the same thing but there is a subtle difference. Goodhart is observing a fact about measures, Campbell is offering an explanation of Goodhart’s observation, that is, why the target ceases to be a good measure. Goodhart is making a very general observation which might be true for a number of reasons. One alternative to the Campbell version is that value of a measure ages so, by the time the measure has become a target, the world has moved on and the target isn’t any good.