Just consider what would have happened if God had NOT intervened to stop the evil that was abounding back then. The tiny number of decent people would have been wiped out by the evildoers.
As for Noah's descendants, sure, badness developed and grew because Noah and his sons were sinners, too. But as Noah "walked with God" there was the grace of God experienced by him saving them from annihilation, and the line of humanity through his son Shem led to the Savior being born centuries later. The Savior warned us that just as it was in the days of Noah so it would be again, prior to His return.
Clearly, the flood account serves as a warning for humanity today. Ignore that lesson from history and you too will perish when the Savior returns. You see how patient God is? You see how He gives ample opportunity to stop being bad and mad (for only a mad person would reject salvation that is free). No. You don't, do you? As Jesus said, "and they took no note till the flood came and swept them all away." (Matthew 24:39)