Companies often measure success with “vanity metrics”, such as Net Promoter Score (NTS) or basic analytics data. They are called vanity because they are not painting an accurate picture of the site’s success, but make the owners feel very good.
UX designers sometimes use flawed metrics too: we think that if a product is viable, desirable and feasible, then it must be successful.
It does not, however, guarantee the product will be ethical: think about Instagram, for example. The app is highly desirable, and it was built purposely to be so, and to increase engagement. However, was it ethical to use psychology to make it almost addictive? You can decide.
Lennart, therefore, recommends adding a fourth dimension, ethical, which should wrap all others, making the product purpose-driven.