Brands have value insofar as the consuming public recognize those brands, right? I'd wager that Diablo is most famous for the first two games which both came out 20 years ago, and that the enthusiast segment of the market recognizes this brand more than the casual segment (who, by nature of being casual, know less about historic games.) In the enthusiast/pc segment of the market, Diablo may still be a valuable brand. But in the casual/mobile segment of the market, the Diablo brand is relatively unknown and relatively worthless.

Trying to sell games to mobile/casual gamers is fine, but they probably should have invented a new brand for the purpose (like they did with Hearthstone) instead of burning an old brand for little gain.