Sure, but assuming everyone puts similar amounts of efforts into manipulating the metrics, the model will come to reflect the fact that those metrics are no lon

Sure, but assuming everyone puts similar amounts of efforts into manipulating the metrics, the model will come to reflect the fact that those metrics are no longer predictive, and there will no longer be a benefit to manipulating them. Thats the benefit of this scheme - it isn't statistically sound, but all the statistical shortcomings eliminate themselves as people try to exploit them.