Both our unhealthy habits, and the "simple rules" to keep us healthy, have been around for decades. Building devices that give us gigabytes of data won't change anything. Dr Peter Attia makes a compelling argument in his book "Outlive" that science, as it is structured now, has achieved miracles when it comes to injuries and infectious diseases, but has been more or less powerless, for entirely systemic reasons, to do anything about neurodegenerative and cardiovascular diseases or against cancer and diabetes. His book is well worth reading to understand his argument--but the gist of it is that those require lifestyle changes.

The person you replied to mentioned diet and exercise, that seems like an area that would benefit directly from this type of scan. Being able to track the effect on body composition in a highly accurate way where we know exactly how much muscle and fat are gained or lost and where that's happening could tell us a lot about not only the effect on the "average person" but for each individual. I'm sure there are many other less obvious things that could be tracked using this technology.