Arthur Morgan would likely feel a deep sense of satisfaction and peace seeing John and Abigail together and happy. Throughout Red Dead Redemption 2, Arthur is shown to care deeply about John and his family, often urging John to leave the gang and start a new life with Abigail and Jack. Arthur's actions and sacrifices were partly aimed at ensuring a better future for them. Seeing them happy and settled would affirm that his efforts were not in vain, bringing him a sense of fulfillment and closure.