Any metric that you choose will have the same problem. The metric may improve over a period because of actions that you take, or they may improve because of something entirely unrelated. This is an important thing to be aware of.
However, there is another problem with metrics. Sometimes, it is too easy to pay too much attention to vanity metrics that won't add anything to your bottom line. Will moving from a 25th place to 1st place for a search help your business? That depends - does that search generate traffic, and does that traffic generate revenue? On the other hand, increasing revenue is never a bad thing.
I'd argue that unless decision makers have too much time on their hands, they're better off focusing on metrics like revenue than metrics like search results. Validity will be a problem across all metrics, but at least there isn't a tendency to optimize useless metrics.
However, there is another problem with metrics. Sometimes, it is too easy to pay too much attention to vanity metrics that won't add anything to your bottom line. Will moving from a 25th place to 1st place for a search help your business? That depends - does that search generate traffic, and does that traffic generate revenue? On the other hand, increasing revenue is never a bad thing.
I'd argue that unless decision makers have too much time on their hands, they're better off focusing on metrics like revenue than metrics like search results. Validity will be a problem across all metrics, but at least there isn't a tendency to optimize useless metrics.