Per Aspera Ad Inferi

Let's break down the evidence

The deal Richard Warren made was to protect himself, his children, his community.

This deal protected Richard from death entirely as long as he is on the island.

We know Richard cares about his children's survival

The thing he makes the deal with said the children would die if they leave the island.

anyone born on the island dies it they leave

none of them were born on the island in the days of the early settlers.

Francis fell off the boat when they were trying to leave and was brought back to the island

There is a painting of a young woman with a missing finger (like Francis) wearing the brooch Sarah gave Francis

I think because Richard is the one that made the deal the Warren bloodline would be more protected than the rest of the community. The immortality part of the deal could extend to the Warren line as long as they don't leave the island. What if the kids didn't immediately die when they left? what if leaving the island just meant they would no longer have that supernatural protection from death? I think the only reason Richard Warren died when he hit that boundary line was because he was centuries old.

So! I think Francis could still be alive and if we consider our cast most of them have grown up together so we know most of them can't be Francis. Francis, if she is alive, would be an old woman and we know only one old woman in our circle of characters. If Francis is alive she is Ruth!