For what it is worth, I am with you old coach. You are not moving the goal posts. Mental toughness can be built over time. Your progression of threshold workouts helps do that. It helps by way of getting kids to understand what threshold pace is, and to help them build there way to sustaining it for an extended period, despite the discomfort they feel during the workout. Once they come to realize that they are capable of maintaining the desired pace (as a result of doing the works as cruise intervals initially), they no longer fear that pain or take it as a sign that they are going to hard, but rather recognize it as an indicator that they are working at the appropriate pace. That ability to work through discomfort is the essence of mental toughness, and your progression approach helps them do just that.