Diet is of primary importance if your only goal is to lose weight. Exercise plays a bigger part than many realize though. And as madgeylou pointed out, losing weight doesn't mean you're healthy. If your goal is to be a healthy human being you should exercise frequently and make good eating choices.
I agree with this and won't quibble about the numbers for running marathons (2/3 of a pound of fat vs. 1 pound of fat).
I agree that reducing fat isn't the only thing that people should be doing to get healthy, but given the obesity rate in the US I think it's probably a more pressing concern. I also think that losing weight will end up increasing people's exercise because being having too much fat generally makes it less appealing to do things like go run around / ride a bike with the kids (or go to the gym and work out when that becomes possible again).
And when people exercise for the purpose of losing weight, it's super disheartening when they don't lose weight. This is probably because they aren't concentrating on what they're eating because the marketing we see is usually about some new fad exercise equipment that's going to get you in shape.
When I was in grad school, I started to run 5 km / workday on the university track. But then, I'd get a Blizzard at DQ 50% of the time when waiting to take the commuter train home to treat myself for the workouts I was doing. This was stupid and somehow I was surprised when I wasn't losing weight.
I largely agree with your post here.
From a purely aesthetic standpoint, people typically don't want to be frail and weakly . . . they want to look less jiggly. When sedentary people start to train they usually start to change their body composition (putting on muscle while losing fat) which reduces waist size, tightens up the stuff that jiggles, and increases the amount they can eat each day without adding fat to their frame. So, you can easily be working really hard and getting discouraged that you don't see pounds drop off at the scale . . . even though you're getting healthier and moving towards looking better at the same time.
That's why my recommendation for people is to record and track athletic performance rather than monitor weight loss. If you used to take 10 minutes to run a mile, and now you can do it in 9 or 8, you have significantly improved your fitness and overall health. In my experience, this type of measurement is much more consistent that a measure of weight (which can fluctuate due to salt intake, water retention, fiber eaten in the past few days, etc.). The beauty of doing this is that you will improve for a while but eventually plateau in your athletic progression if you stick with a poor diet . . . but by that time you're already living a healthier lifestyle, and the dietary changes can be introduced in a more gradual and measured way with a clear performance goal rather than a (typically arbitrary) weight goal that is subject to many confounding factors.
Another benefit to pushing exercise is that for the vast majority of people, exercise is more fun. When you diet and limit calories*, your body usually punishes you and makes you feel like shit**. You feel like shit because you are literally starving yourself. When you move and exercise, positive mood-altering endorphins are released into your body that make you feel better about what you're doing. When properly scheduled as part of a routine, you can use this mood enhancing, mildly addictive trait of exercise to create and enforce this beneficial habit.
* Not talking about avoiding cake, cookies, brownies, sugary drinks, potato chips, donuts, candies, ice cream, etc. - if you're not already limiting these pretty hard and are trying to lose weight then you're an idiot. Cutting this sort of poison out of your life usually makes you feel better. I'm talking about people who are getting 90 - 95% of their calories from real food and then try cutting 250 - 500 cals out each day.
** The exception being if you go really, really far overboard - most people who are anorexic starve themselves for so long that their bodies give up on releasing ghrelins to make them feel hungry and just pump out seratonin to try to make what it sees as your last few starving days on Earth more tolerable. But very few diets I'm aware of recommend this level and type of extremely dangerous eating behaviour.