A lot of the fanbase got very attached to Arthur Morgan and still would like to play as him after the main story. A solution to this would be adding the option to switch between the two characters like how it was shown in GTA 5. 

This would make the fanbase happy and it would give them the chance to complete missions they missed instead of restarting the entire game. It can be argued that Arthur Morgan was the best protagonist ever created by Rockstar games and players all around the globe would love to continue their experience as Arthur Morgan.