Let's assume for a moment that the Bible (and the flood account) would be true, would be God's word and 100 % accurate, and even that JW's were right in their interpretation.
What happened after the flood?
Shortly "After the flood of Noah’s day, his great-grandson Nimrod became a tyrant, 'in opposition to Jehovah'" (Good News to Make You Happy, page 91)
352 years after the flood Abraham was born, and in his day the world was already "steeped in Babylonish idolatry" (Insight, vol 1, p. 29)
1000 years after Armageddon the number of godless people will be "as the sand of the sea" (Revelatoin 20:8).
Even after that, "anyone that carries on a disgusting thing and a lie" will not be allowed to enter the New Jerusalem, so it seems that such people will exist (Revelation 21:27).
So, according to the Bible and JW doctrine, God killed almost all humans in the the flood so that only righteous ones remained, but few years later the world was filled with unrighteous people. He will kill almost all humans soon in Armageddon, so that only righteous ones will remain, but at least 1000 years later the world will be filled with unrighteous people. He will kill these again, but even after that there will be unrighteous people.
So, if God exists and the Bible is true, he did not accomplish anything with the flood.