From the point of game design, the detail from better RT lighting is actually a negative. For visual clarity the game benefits from soft shadows, which means large area lights, where RT isn't so beneficial.
RT in a game like this probably wants a photorealism approach, so uniform GI and very artistically chosen lighting. Once you go stylised like D4, the technical choices made to support that art were pretty ideal and not limited by tech. Basically, Blizzard had already won the visuals. My only complaint with the game is the light source stuck in front of the player casting their shadow backwards, so as you turn around your shadow spins, and clashes with the orientation of other people's shadows.