What's so horrible about focusing on an aspect that people enjoyed? GTA V focused on heists because people always liked those missions the best in the other games. If the entire industry suddenly moved towards Naval combat games I'd think you'd be making a good point because that's what happened with FPS games for a while.
If it was up to me I would have just got rid of the Assassin aspect and made it a pirate game though. Sometimes its best to move away from a franchise for a bit, or just not make it too much of the same old.