But when you say something has half the nutritional value it had, then shouldn't there be less of everything, not just the smallest group?

Imagine reducing the fiber, sugar, water, minerals, and "everything" else in some apple slices by half. You haven't made it less nutrient-dense; you've eliminated everything that makes it up; i.e., you've just made it smaller.

The researchers aren't arguing that we eat less. They've defined some things as important "nutrients" in some sense and noted that the relative (not absolute) concentrations of those have decreased.

Whether that matters for a particular application is up for debate. Certainly macronutrients matter. It's really not hard to understand what they mean though.

For the average person, this pushes the incorrect narrative that somehow our food is becoming drastically less useful. The primary use of these crops has always been the carbohydrate content, followed by micronutrients; all this really says is that we should still have a diverse intake of food to reach RDA’s of said micros.

That may well be true, but that disagreement at least bothers to engage with what the authors were obviously saying, whereas the former posts were an exercise in pedantry. It's actually a point worth bringing up, which is commendable.

Secondarily, I disagree with you a bit I think:

(1) Wheat is an important iron source. Lack of vitamins minerals in modern rice definitely killed a bunch of poor east-Asians who couldn't or didn't know they should supplement their diet more heavily with non-rice foods.

(2) Even if the focus on cereal grains were the main point, an obvious side-suggestion is that cereal grains would not be the only afflicted crop. At a minimum, that would need more study. Some of that study has been done though, and potassium and iron (among others) have significantly declined in other common vegetable crops this century.

(3) A diverse intake of food absolutely does not suffice to reach RDA's of said micros. Just to leave things brief in the context of short interweb comments, (a) how would you hit an RDA of calcium in a day's worth of calories and without additional supplements, (b) is that diet "diverse", and (c) does that plan leave enough budget in your available calories for the other micros you'd like to target? Calcium isn't the only micro which is hard to satisfy with a generic "diverse" or "raw" or "natural" or whatever diet, and in 2000 calories (less as you age or if you're smaller) it's very, very difficult to hit all your micros with any combination of commonly available foods, especially if your strategy isn't to explicitly make sure you hit those RDAs.

If there was a 10% increase in one area and an 80% decrease in two others I would still be okay saying half as much. And then there is some wiggle room in how you weight the categories.

(But that's not what's going on here)