Making the destruction of humanity and Earth by the Deluge a moot point. That’s a pretty big oopsie, but I guess God would be up to it. If God made oopsies.
I guess you're focused on sinful nature as though it's passed genetically (thus the flood was meant to wipe out the sin lineages). I'm seeing it as who's going to raise the post-flood people (righteous Noah to raise them instead of the more blatant sinners who died in the flood).
I guess either way the proof was in the pudding. The sinful tower of Babel (Genesis 11) was just a couple of chapters after the flood. Evidently some of the sinful nature survived on the ark. And of course we know that from the rest of the Bible, including that Jesus had to die for our sins.