Earlier quoted context omitted.
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…
Diablo 3 does not have a recurring fee so it is not relevant how many active players it has; the only relevant metric is units sold.
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.