You beat any game in 4 days and you'll easily get burnt out.
The sandbox play is where the replay value lies.
I do agree on the side quests, but all games are like that for the most part. You gotta scatter some story, some side quests, and sandbox play. That's how the game stays fresh.
Same can be said for any single player open world game.
I'm on the second island, and haven't progressed through the story, because I wanna have it last.
Here's hoping you can enjoy some screwing around until some dlc is released.
Another island with maybe some lions or deadly predators will make me happy.