The post-Flood scenario was largely the same, with one glaring exception: the earth was now to be populated by imperfect humans. I'm not sure what God's thought process was. He knew things would decay again.
So in essence, it may be that God made the same 'mistake' not once, but twice, if the Bible is to be believed.
Actually, one reason I have trouble accepting the Genesis account of creation and the fall, and then the Flood, is because it seems to defy logic. It's also too simple, too convenient. It seems awfully convenient that the first man and woman sinned. By convenient, I mean from the story-telling standpoint. In other words, it would seem more realistic if Adam and Eve would have first had a few children, and one of them would then sin.
But here we have the first couple, supposedly perfect and full of procreative power. And yet strangely, she does not get pregnant until after they sin. Just seems too convenient.