What we know for sure:
It brought in a lot more players, be they old or new.
Critically, it's being praised.
Players seem to be enjoying the PvE side of things bar some slight annoyances.
Decline in activity is being reported on every single major 3rd party tool, this is normal, however, it's all being reported as much faster and steeper than previous post expansion states.
Blizzard's stock price has fluctuated a hell of a lot during the few months mists has been released, including an all year low in mid November with current prices closing in on this figure again. ****Remember Blizzard's stock is Activision Blizzard stock, all their products influence what goes on here****
Speculate on those facts as you will, but they are all true.