I always hesitate to speak on these income issues since I do almost all nonprofit work, but on the other hand I am experienced in business and my sites that assist me financially work, albeit a little differently than most (I only need and can only use a few clients to fill my available time for my paying sites). But I do have a little knowledge and understanding of the issues you are looking at.
Your observations have merit. However, there are a couple of ways you have to work around them and I suspect that almost every successful site does it one way or another.
One way is making your site a little more "long tailed." You are looking at topics as either highly profitable or not profitable. The idea is to have a field that is high paying (say dieting) but to pick a more specialized approach, for example one that might not be too crowded yet (maybe it is but this will give the idea) would be helping over weight children.
Another approach is to take the fields that you are looking at as low paying and come up with a way to make them pay better. For example, if you can get a lot of traffic, even if the Adsense clicks are pennies and there are no Amazon purchases that pay much, perhaps you could write and sell your own book on the topic. You may well be able to come up with some product that would generate a good bit of income in almost any field.