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"?
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...
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.
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.
They decided no with D3, as did I (I played D2 for probably 7 years...)
How else would you judge a piece of art besides "I personally didn't like it?"
It would be seriously lame to like a game just because it made money or some critic told you to.
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.
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.
RoS sold pretty well. (2.7m in first week: https://web.archive.org/web/20140530093934/http://www.euroga...)
But since RoS is an expansion, rather than a sequel, it's fair to ask whether that's apples to oranges. I don't actually know.