Diablo 3 sold 3.5 million units the first day and 30 million units total as of 2015, so I'm not sure by what metric it would be considered a failure... aside from "I personally didn't like it"?
The relevant metrics would be daily/monthly active useres (DAU/MAU) and churn rate. But Blizzard doesn't report those number's unless they're good (e.g. Hearhstone/Overwatch hitting new record MAU last year), and hasn't ever released any such numbers for Diablo 3.
This article[0] claims Blizzards overall MAU was close to flat YoY-Q4 2016-2017, knowing that Overwatch and Hearthstone are hitting records high MAU, while overall MAU is flat means that the other games (D3, SC2, HotS) are losing players.
It's a success in the way Matrix Revolutions was a success, massively profitable[1] yet a disappointment to fans (see diablo 3 fan ratings[2]).
That's an arbitrary statement. The only relevant metric is revenue outside of whatever bars you would like to matter. Over time, units sold is a poor return and D3 was disappointing against every prediction beyond units sold. It was a disaster, in comparison to the recurring success of PoE.
> The relevant metrics would be daily/monthly active useres (DAU/MAU) and churn rate.
This metric doesn't really make sense for a (mostly) single player game like Diablo. Once the player has finished the game they're unlikely to return. For a game released six years ago of course we would expect the current active player base to be very small - most of them have since moved on to other games.
The fact that D3 outsold D1 and D2 combined suggests that while it might have been disappointing to some subset of players, clearly they managed to find a much bigger audience that enjoyed the game nonetheless.
You are still comparing sales to game engagement and longevity.
You also need to shrink the amount of D3 sales to compare it to D1/D2 sales because the game market has grown wildly since they released. Then adjust that because it was a sequel a decade in the making.
It's like comparing the star wars sequels to the original films. The nominal values are way bigger, but if you adjust them for inflation they might tell a different story.
I think it's hard to find an objective measure, given that there are so many franchise fans that solid sales are basically a given - I wouldn't expect to see the consequences play out until D4.
That said, the fact that they rebuilt the absolute core gameplay loop - loot collection - to incorporate a real-money auction house, and had to roll it back, speaks volumes. Business-people do not abandon a monetization strategy unless it's proving to be costly in unintended ways.
The "consequences" were the sales of the $40 Reaper of Souls expansion to Diablo 3; if players were so put off by the core gameplay loop, even the removal of the RMAH wouldn't bring them back.
Many Diablo players have said they will never play another game in the franchise afterwards. I know about 5 people who bought it, played it for a few hours and promptly returned it for a refund. For some the laughable event of leveling up and having nothing to select or choose even if you went into the options menu and enabled the "Advanced Level Choices" option was a far cry from the enjoyment of getting your first level in D1/D2 and seeing all the choices you had.
This article[0] claims Blizzards overall MAU was close to flat YoY-Q4 2016-2017, knowing that Overwatch and Hearthstone are hitting records high MAU, while overall MAU is flat means that the other games (D3, SC2, HotS) are losing players.
It's a success in the way Matrix Revolutions was a success, massively profitable[1] yet a disappointment to fans (see diablo 3 fan ratings[2]).
[0] https://venturebeat.com/2018/02/08/blizzards-monthly-active-...
[1] https://www.the-numbers.com/movie/Matrix-Revolutions-The#tab...
[2] http://www.metacritic.com/game/pc/diablo-iii