In the computer industry, if it has the elements of an RPG but didn't have a complex inventory to manage, it typically was considered "An Adventure Game"
On the question of is Diablo an RPG, it is if you consider figuring out what treasures to put in your backpack to maximize the value you get out of your dungeon run more quintessentially an RPG than having a soap opera with NPCs. I'm not judging if that is wrong or right, just saying that based on what is called an RPG historically, it might be that a backpack with limited space in it to utilize is more essential being an RPG than dialogue.
Think about the game play experience of Nethack. World of Warcraft. Skyrim. Mass Effect. Mass Effect is particularly interesting in that after the first title you had much less resource management both Chargen resources and inventory, and the later titles were much less like RPGs.