Whether people want to admit it or not D3 abandoned D2's formula and adopted WoWesque simplicity and accessibility. This was done as part of Blizzard's larger strategy creating highly accessible "sticky" games that attract and retain lots of players for years.

Now, there is nothing wrong with simplicity for the sake of accessibility. In WoW it is arguably necessary for the purpose of making balancing easier, but most importantly ensuring that players can eventually reach all the content they work towards. WoW's model is based on constant content creation and consumption. The issue is not realizing this doesn't port over to games that don't regularly see new content, lack competitive incentives/challenges, and has for less social interaction with other players. WoW wouldn't have been much of a success had players been left to farm the same end-game content for a year let alone two.

Yet, D2 has had incredible longevity without such updates. The question is why did D2 fair so well when D3 is struggling and the answer comes from the single most misunderstood thing about the Diablo series. Diablo is not about finding loot. Saying Diablo is about finding loot is like saying Magic The Gathering is about collecting cards. The real basis of playing these games are builds. Theorycrafting, building, and testing characters/decks. D2 revolutionized the genre because it opened up how players could make choices regarding how their characters played. Obviously, all choices weren't equal as there were plainly wrong ones, less optimal ones, and eventually cookie-cutter setups, but the point was that complexity added both an increased number of choices for the player and distinct play-styles between these choices.

In WoW players don't ask themselves what kind of Mage or Warrior do they want to make because those choices have largely been removed and replaced with predetermined setups for the sake of equal footing in a content consuming community. Players really just level and gear their standardized classes. This is why this simplicity undermines what makes a loot based ARPG enjoyable long term.