That's a bad rationale. Mediocre performers, are, by definition, average. Unless your hiring process has completely changed, you're not likely to get anyone better if you toss out a mediocre performer and hire someone new: the new person could be better, or could be worse. Moreover, the mediocre person is experienced and productive; the replacement will not be, and probably won't be for a long time. (If the mediocre performer isn't productive, then they're not mediocre unless most of your employees are terrible, and should be fired for bad performance.)