Hmmmmm. That's interesting. The dirt doctor claims, "from the article you referenced", that
DE will not hurt earthworms or beneficial soil microorganisms.
Maybe the earthworms just tunnel away from it, otherwise some of these claims could be a bit contradictory, at least on the surface.
Fire ants are becoming increasingly scarce around here, too, but they aren't quite gone, yet. All the flooding that we had in 2007 killed a lot of them, because they weren't able to get their broods high enough to avoid the water, but I don't see why they couldn't have eventually recovered from that. Also, they lived through some worse droughts than this, in 1984, 89-90, and 96. We appear to have some "new" ant species living here, now, and I wonder if they might be eradicating the fire ants. I also wonder if the fire ants might have succumbed to a native disease, that the native ants have resistance to.
I've noticed at least one big disadvantage to losing the fire ants. A few years after the fire ants moved in, all the deer ticks disappeared - the fire ants got 'em all. Now that the fire ants are becoming hard to find, the deer ticks seem to be back.

Whatever it is, that is exterminating the fire ants, it isn't something that we are doing to try to control them, because I've almost stopped "treating" them, and they are continuing to decline, here, just the same. As you pointed out, we have never treated the pastures and fields, (which amounts to at least 99.99% of their habitat), but they are almost completely gone from there, also. 10 or 15 years ago, you couldn't walk in some pastures, without stepping on a fire ant mound. You couldn't pull off the road to change a flat tire, because your boots or shoes were guaranteed to be covered by swarming fire ants, within a few seconds after stepping out of the car. Those pastures are virtually free of fire ants now, and so are the roadsides. So far, though, I haven't run into anyone who has the foggiest idea why.
We're just mighty lucky that those ants were "imported", or the EPA would already have this entire area in a stranglehold, after declaring it "prime habitat for an endangered species".
Tex