Most of us are probably eating healthier than 40 years ago (Heart disease is way down), so not sure it's diet. Unless of course its all the chemicals and contaminants that we're getting with our food...
I used to believe we eat shit, but my ancestors were really poor and basically ate just a few food types most of their life. I think I've come a long way and am convinced that, even if I were to eat a very occasional McDonalds, it'd still be eating a lot better. It's a lot more diverse and that really makes a difference. Don't know about you but I very rarely eat the same thing two days in a row.
This is why I think that jumping on the "BPA must be the problem!" answer to low count question is a bit ridiculous, especially when people are becoming more unhealthy by the day. People who eat more processed sugary foods and drinks that come in BPA containing packaging are also more likely to be obese and unhealthy too.
Lacking physical work also does not account for all of the weight gain we are seeing at the moment - obesity rates remained stable in the '60s and '70s when we lived similarly lazy lives. (1)
1. http://www.nejm.org/doi/full/10.1056/NEJMsr043743#t=article
I don't see how food being diverse makes it healthier though. You diversify against risk, is there risk here?
Protein bars, granola, multigrain, fruit juice, don't have high fructose corn syrup, just have a bunch of sugar instead, it's more natural!, etc and having sugar and wheat and soy and other crap put into almost everything you buy at a supermarket has contributed to this mess.
My general rule of thumb is if you're not having at least 3-4 solid servings of vegetables (not fruits, just veggies), every day, excluding corn (corn should be stripped of its vegetable status), you're probably not eating anywhere near as healthy as you should be. Fruits don't need to be avoided, but you shouldn't think you're eating healthy if you're eating 5+ servings of them every day, since they do have sugar in them (offset a bit by the dietary fiber, but still sugar).