What are the long-term consequences for John's health and wellness if he continues to use this technique

Ergonomics is basically a study of how people do their work in the work environment.

Explanation:

For example, the position that they sit, the number of breaks that they take between the work, posture, etc. There are certain health issues related to continuous use of gadgets.

The improper posture may lead to back pain which may lead to life-long back bone issue. Watching continuously the work on the system may spoil eye sight which may lead to eye-dryness and wearing spectacles at an early stage.

In order to avoid health issues, follow exercises, take break as required, maintain correct posture, etc.