Never really got that either, considering as to how:
A ) The number of players is in no way an indicator of the games quality, just look at Runescape, around 1 million subscribers and over 10 million active accounts including the f2p part in 2011. Does the 10 million players make it a high quality game? I highly doubt it.
B ) Sharded servers make the presence of such number of subscribers useless anyway, it doesn't change anything to the fact that the server population I'm playing with is just a few thousand, nor does it truly have any effect on the instanced content, even when cross-server, considering as to how pop times to that doesn't change much the moment the pool is just 100 people, or whether the pool consists of 100 000 000 people. It's the ratios between the different specs which makes the difference.
C ) More monthly income to the publishers as said is good for the shareholders, but that in itself also doesn't have to come for good of the subscriber. As they will be more inclined to use such money for side projects, unrelated games as there's a limit on how much you can invest into an MMO without overwhelming the players anyway. Yes, there's a thing as too much.
I think the biggest problem simply is that too many people are too cought up with the entire WoW subscriber numbers, it becomes the leading argument in any random discussion. Game quality or potential is quickly forgotten.