There is no source supporting your claim of nutrient decline in that magitude thanks to soil depletion. It's mostly due to modern crops that grow tall fast, and are thus mostly made up of water.

Some argue that it’s instead because we’ve promoted food strains that have more sugars and less nutrients, and I’m still studying that, so I have no position to offer about it yet. Brussels sprouts is a good example of doing this in a way that doesn’t damage the nutritional value, but the general U.S. avoidance of anything pungent or bitter is reflected in having bred out all of the ‘unattractive’ nutrients from our food strains. A good litmus test for this is to check for dandelion greens on the shelves; if present, the market likely sells a broad spectrum of produce that isn’t simply designed to be a sugar bomb; if absent, I’d be shocked if you found anything nutrient-dense at all.