Why did God say he would never again curse the ground (Gen 8:21)?

“Very well,” one might ask after reading the latter answer, “why not? After all, God just said that ‘the intent of man’s heart is evil from his youth’. Why not continue with the cursing and destroying of man?” The answer, I believe, is that the curse of the ground, i.e., the curse for Cain’s crime, for which God appeared to turn his face away from a whole line of people, has as it were “played out.” That line is destroyed; the world is reborn, and the reborn world is given a law and a covenant. It is as if God says, “See what you can make of this, ye hapless men with evil hearts; at least I will not utterly destroy you or curse the ground as I did Cain’s line, which was unusually wicked.” But since soon some (such as Canaan) were to be cursed (9:25-27), albeit by Noah, not by God—it seems God respected the curse. This makes it particularly clear that God means that all of humanity would not be cursed, at least, not as long as “the earth remains.” It is significant that God, in the end, will indeed destroy the earth again, as he utterly curses and destroys his enemies forever (see Rev 21:1 and its context). The time of clemency that began with Noah will not be forever.