I was with you up until you suggested substituting “mass” for “weight.”
Weight and mass have a nearly 1:1 correlation, especially if you’re moving at sub-relativistic speeds across the surface of a planet. Besides, when you step onto a scale, the scale does, indeed, measure weight. If a scale displays your mass in kg instead of your weight in pounds, that scale is making several assumptions in order to convert the amount of pressure pushing down upon it (in pounds or Newtons) into a measurement of mass.
Besides, “mass” isn’t much better than “weight” - Capability is probably a better indication of health (Can you run X distance without stopping?), but that’s much harder for someone to measure on a day-to-day basis. Waistline is a good one, but then you might have people squeezing into corsets and claiming that makes them healthier.
Weight - excuse me - mass is (if you make the same assumptions that your scale does) one of the easiest things to measure, and has a decent correlation with fitness.
