THIS Is The Perfect Red Dead Movie
1. The Legacy
John and Sadie both played significant roles in building each other's new lives. Sadie helped John provide for his family after the gang collapsed. In return, John enthusiastically offered his home to her, helping her get back on her feet. She lived with them for a time, and she knows full well what John, Abigail, Arthur, and even Uncle wanted for Jack.
Abraham Reyes, meanwhile, represents one of John's greatest failures. For all his crimes, John's role in the death of Colonel Allende and the deposition of President Sanchez had the greatest effect on the most people. He enabled the tyrannical Reyes to rise to power, which means that every life destroyed by Reyes comes back on John.
Wouldn't it be poetic to see Jack thrust into the middle of his father's mess, forced to clean it up with the help of the only "family" he has left: the woman who helped his father build the life that was taken from him? Conversely, since Sadie was the one who led John to Micah - indirectly outing him to the Pinkertons and kicking RDR1 into motion - wouldn't it be poetic for her to make it up to John by saving his son?
You're damn right it would.