That doesn't explain the timing.
No sin nature is necessary for sin, just ask Adam. Why not destroy Adam and start over, or Cain. If God was working to destroy a sin nature in man He failed to change anything by letting Noah and his sons live and commanding them to breed.
The question really isn't about sin nature though, but about timing. Is it only a coincidence that Adam died only 721 years before the flood, and during the lifetime of Noah's father? My thoughts were that loosing his leadership might have contributed to the general condition. I wish we had a more detailed account of these years.
If not the patriarchs, what other reason is their to wait 1651 years to destroy the earth with a flood?