Diablo 4 is a game designed to feel like a real single player Diablo sequel to 2, with multiplayer features on top. Its dungeon, bosses, gearing and build system are still done with the regular diablo approach.

FFXIV is a MMO that adds superficial single player elements. It drastically tones down or removes a lot of what makes the gameplay of a single player RPG for sake of still being a MMO, like its job design, dungeon design, map design and MSQ difficulty.