This is desirable and it's easy to explain why. For any student there will be a range of grades they could expect to achieve. The fairest grade to assign is the

This is desirable and it's easy to explain why. For any student there will be a range of grades they could expect to achieve. The fairest grade to assign is the best case grade in that range (not the midpoint), because there is no evidence that they would not have studied hard and achieved that grade in the exam. If every student is given their best case grade then you will indeed see an overall rise, this simply reflects a lack of knowledge of who would have studied hard and who would not have.