It's a neat show, though I agree that much is speculation. They can state that, absent people to operate the locks, Chicago will flood, or that refineries will eventually go boom, but the timeline for some of the stuff is pure speculation, especially once they clear the six month or one year point.
On the other hand, yeah - parts of Detroit (and other areas in the rust belt) do provide a nice example of how quickly this is true:
Ozymandias
by Percy Bysshe Shelley
I met a traveler from an antique land
Who said: Two vast and trunkless legs of stone
Stand in the desert. Near them, on the sand,
Half sunk, a shattered visage lies, whose frown,
And wrinkled lip, and sneer of cold command,
Tell that its sculptor well those passions read
Which yet survive, stamped on these lifeless things,
The hand that mocked them, and the heart that fed;
And on the pedestal these words appear:
“My name is Ozymandias, king of kings:
Look upon my works, ye Mighty, and despair!”
Nothing beside remains. Round the decay
Of that colossal wreck, boundless and bare
The lone and level sands stretch far away.