People bought it and ran, bananabum21.
They did not
That 1 million player per month was as of Feb this year. One year after release. With 7.5 million unique players total
At June 2018 uni announced that 3 million new players joined in the "past few weeks"
As of October 2018 there are 15 million registered users
The game sold shit loads, is receiving amazing and lots of support, keeps increasing its player base, and is still getting better and better. It's a huge success for Ubisoft