This suggests modern civilizations need irrigation to feed themselves through agriculture. But this doesn’t relate to whether such reliance will lead to collapse from toxic soil.

Even if other factors would have led to the Sumerian collapse, that doesn’t change the fact that toxic soils contributed to the Sumerian collapse or whether what happened to the Sumerians will happen to modern civilizations.

This points out why what happened to the Sumerians might not happen to modern civilziations. Modern techniques might not lead to toxic levels of salt and impurities in soil.

We’re concerned with what happens to the ones that do rely on irrigation for agriculture. But whether these kinds of civilization are common or rare doesn’t relate to whether these kinds, however many there are, will collapse.

We have no reason to think modern soil doesn’t also contain some toxic salts and other impurities. In addition, even if modern soil doesn’t already have this, that just means it might take longer for the collapse. But (E) doesn’t suggest there wouldn’t be a collapse.