The men of renown as described in the Bible were genetic hybrids of fallen angels and women. They are not redeemable as they are genetically not human. They proceeded to bring about terrible harm to humanity at the time and to the environment. They were giants whom fed on human flesh and bone, and demanded worship from humans.

Their deeds and behaviors have yet to be forgotten to history as they are still known by the unofficial accounting of their actions. You can always look to ancient mythology for a better understanding: Greek mythos, Indian sub-continent mythos, Native American mythos, Chinese mythos, Aztec mythos, Babylonian mythos, Scandinavian mythos, Germanic mythos, Egyptian mythos, etc.

Everywhere you look you can see their memory is still retained as they were exceedingly terrifying and destructive to humans and to the rest of creation. Noah's bloodline being found pure in his generations (genetically unblemished by nephilim dna), was chosen by God to carry forward the torch of humanity.

Did you ever wonder why God was so strict with the Israelites about not mixing with other people's (preserving genetics and bloodlines chosen for the coming savior (Jesus) from a similar attack vector is the answer).

Did you ever wonder why God ordered the Israelites to kill every person and destroy everything of the Canaanites, the Amelakites, and other people groups? The Canaanites were known to be giants, the scripture gives indications that their genetic bloodline in at least some of their people was polluted again by Nephilim, the valuable animals of their people's were also required to be slaughtered for the same reason.

God was not going to allow the Nephilim to simply start again with destroying the creation permanently and irredeemably. They were restrained and where pockets remained, God raised up tribes of people to destroy them. Many groups have such stories of fighting giants that would eat human flesh.

Don't think I'm arguing for some human genetic purity test or something like that. If you are human and your DNA is part of the human genome, then there is no problem. If genetically you have DNA that was somehow altered to be no longer human (science or via fallen angels/ nephilim) then unfortunately you are a hybrid creature not of God's creation and thus the blood of the Lamb would not be extended to cover you.

Does anyone in this world today fall under the hybrid designation? -- I have no idea and will allow God to make that judgement.

Is it possible that someday science will take us past a point of no return with genetic alterations? -- I would say this is very likely to occur within the end times. A Mark of the Beast that is a genetic modification would make allot of sense explaining why those who take the Mark cannot be saved.

Where is this scientific genetic modification line? -- I cannot say for sure. I believe the modification would have to involve synthetic genes that are not found in the naturally occurring genome of humans.