I really don't want to get into a big debate here. I feel a game like this should be about 16-20 hours long at least, especially brand new.
I remember playing through Resident Evil 4 on the PS4, it had so much cool stuff. After beating the game, I got two Ada Wong stories that were like extra, not even DLC. Plus lots of other cool extras. I was so excited about that. It felt like that game was totally worthy of a new purchase.
The Arkham games, even after you do all the campaign and extra side quest stuff, you can continue exploring more of the sandbox and find more easter eggs. You also get to play through unique challenge maps for objective based challenges. The PS3 version gave you the Joker for challenge maps. Arkham City gave you Robin, Nightwing, and Catwoman. Granted really just Catwoman who was the free pre-order bonus, but there was plenty in that game already.