Let's suppose that there are increasing obesity rates, to an extent that is genuinely unhealthy. (Aside: I'm 6'1" and my "ideal" weight, according to measures I've seen, is between 150 and 189 lbs. If I'm 6'1" and weigh 150 lbs, I'm not healthy -- I'm frighteningly thin. 189 lbs is pushing it. So, depending on what scale is being used to measure obesity, this may all be smoke and mirrors.) It doesn't follow from this that decline in interest in phys ed is a bad thing, as long as it doesn't also lead to a decline in overall interest in physical activity. Further than this, though, I would suggest that phys ed classes are responsible for a decline in interest in physical activity.
Let me describe a common scenario. Slightly unathletic child or teenager enters phys ed class, wearing a distinctly unflattering gym uniform. More athletic members of the class clique together and chuckle amongst themselves. The former is picked last for teams, is regularly exposed to low-grade public embarrassment, and does not achieve well in the eyes of the jockish teacher. The latter get to pick the teams, are regularly lauded for their achievements, and are graded well by the teacher. Two questions: who is going to develop and retain an interest in physical activity? And who is in the majority? Of course, the answers are, respectively, the jocks and everyone else.
If there is any real interest in solving problems regarding lack of physical involvement of children, rather than posturing for ignorant corporate "reporters", politicians should be focusing their attention on getting kids to enjoy anything that involves physical activity, not just gym class. Gym class is the kind of structured, institutional environment that, without careful nurturing by a skilled teacher, will lead to hatred of the subject-matter. Anyone who's had a lousy science or math teacher knows what I'm talking about; the same applies to gym -- except that there are far more lousy gym teachers, who favour competitive success over health and physical improvement, who would rather give good marks to the winners in a football game rather than reward students who are genuinely trying to improve their well-being.
However, there is no reason to get a gym teacher involved. Community physical activity programs exist, ranging from archery to karate classes to ballroom dancing; thus, as long as a program is reviewed and fits within the standards expected of a physical education class, why should it not serve as a high school (or lower level) credit in its own right? Why, in short, should we let ignorant teachers and the contingencies of institutional learning inhibit something as allegedly important as the health of children and teenagers?
(Of course, this could be generalized for any subject that is considered valuable for children to learn. I would hesitate to extend this past the high school level, if only because the experts in the requirements of the subject are identical to the ones teaching it -- there are no external reviewers who have any business telling professors what they should and should not be teaching. But, if math and science are suddenly supposed to be important -- as one will often hear they are -- then why not allow children who are not succeeding (or developing an appropriate level of lifelong interest) to seek other avenues of learning outside the institution?)
The answer, of course, is that this really is just lip service and band-aid solutions. No one really thinks physical activity is valuable. They just say they do to win elections.