I see what you're getting at. I don't see this as being driven by a change in population over time; this is just bad interpretation. As I mention down thread (and I'm sure you're aware), body builders often have BMI values that fall into the "morbidly obese" range. This is just due to the measure itself, not a change in population averages.

Bodybuilders often have BMIs that wrongly predict their health outcomes. They know this, though, and it's pretty obvious that they're not actually overweight. The problem is the average person, because BMI will list them as incorrectly healthy.

If you're a "normal" weight, but 25% bodyfat, you're not in good shape. But you may think "oh, I'm okay" rather than "I really need to adjust".

It seems obvious to me that BMI will tend to error by understating the problem. That means that if average BMI is increasing, the problem is getting worse much more rapidly than you might think.

Why not just use body fat % measurements?

Primarily because they're hard to take accurately. Using calipers takes training; the bathroom scales which report body fat percentage are using an approximation based on conductivity which have varying results. BMI is quick, and I agree not a very good measure, particularly if only used on their own. For example, lean, muscular people can have BMI values which, outside of context, place them as morbidly obese. But, because they're expedient, they've continued to be used.