Diablo was much more of a slot machine than Fortnite is imo. Neither come close to being as much of a slot machine as watching a baseball game as a fan. Or participating in fishing.
That didn't stop people from playing it 28 days straight.
Oh yeah, Diablo was certainly engrossing. However I think there's a distinction between creating a fun game that happens to be addictive, versus architecting a game to be as exploitative as possible.