I don't think that's a good way to assess if a game is good, there are a ton of problems with doing that.
What I think you are saying is that the multiplayer while it was good didn't do enough to keep people interested.
I would agree with that.
What issues do you have with that being a way to assess if a game, a game where multiplayer is most definitely the dominant side of the game, is considered good by people?
400K people down to 18K in under a year. Sorry but that screams 'Bad Multiplayer" to me.
Oh and you know, playing it for myself definitely tells me that this game has major issues. Mainly because they sought the casual CoD crowd, but problems nonetheless.