Is the subject of cadets to be considered on topic on this site?
Yes, it should be. We are talking about army cadets, one could be excused for thinking they're included by definition.
There have been multiple proposals on the subject (military, weapons, strategy etc). and all have failed to my knowledge. There is little room to cut out potential avenues of contribution.
The life and issues of military cadets and midshipmen should be on-topic. In particular, the several service academies are part of the greater active component military system.
Additionally, I would think that ALL officers in training should be on-topic -- regardless of whether they are labeled as cadets or not. All the various ways in which someone can enter the military should be on-topic. (Quick aside: Initial training for enlisted persons would be included.) Specific to commissioned officers are various avenues in which one be enter military service. As an example, a doctor does the same things in the military and in civilian life. They can be directly commissioned into military service with only some very short initial training. All these aspects of uniformed officers affect all military personnel.