Numbers themselves do not matter
Random thought for this morning: The number of people playing a MMO does not in itself really matter. It doesn’t matter that one game has 11 million subscribers and another has 150k. What matters is whether or not those numbers sustain the quality of gameplay that the developers intended the game to have when they designed the game. Some games need more players than others. There are free DAOC shards out there holding around 900 players online at a time and it feels perfectly populated with plenty of action — the quality of play is sustainable. WoW actually needs more numbers and this showed when they added the dungeon finder. They had millions and millions more than anyone else playing but it still was not enough since the game was designed to need more people to be played the way it was intended.
When numbers do start to matter is when they dip below that threshold and quality of play is no longer sustainable. If you can’t find a group because there is no one to play with, can’t find a city to attack, have no one attacking you, can’t find people to buy your wears, or can’t play the game the way it was meant to be enjoyed then this is when it becomes an actual problem. This is when server tranfers happen for free. Take Rift for example. Trion will offer free transfers in patch 1.3. I’m hoping for them that players choose to all migrate onto fewer servers, which is likely going to be the case, so that the quality of play increases for everyone. Rift doesn’t need 11 million — it needs exactly enough to maintain its own quality of play. I think this is why there are still people playing Warhammer Online. The people who do actually enjoy the game are now on one or two servers (?) and I’m sure the numbers are just fine for them. That’s probably not ideal for the business side of the game but for the game side (what I care about) it’s probably great.
To reiterate, numbers only really matter (to the player) in the context of how they affect the game’s playability.