The fact that people keep saying MMOs need millions of subscribers blows my mind when XI alone was phenomenally successful as well as Everquest and iirc they both only capped out at 500k.
Millions of subs on the scale of WoW on a P2P MMO probably won't ever happen again, even for something Blizzard releases in the future, pretty sure Activision-Blizzard knows this and so did Vivendi when they were trying to offload the company stocks a while back.
It is what it is though, WoW is just one of those massive profit whales, the question is simply how long will it go now. XI has been profitable for 10 years with a fraction of the subscriber base, so it's pretty safe to assume it's gonna be a long while yet. Hard to imagine where Blizzard would be today if they didn't create WoW.
The reason why WoW has so many subs is no mystery: it has an extremely competitive endgame in the form of dungeon raiding and PVP. People all over the world compete to see which guild will be the first to kill a boss, or which PVP team will win a tournament.
The only other pay to play MMO with such a significant endgame
was Rift, which boasted almost 3 million subs at its peak. However, following a year without endgame updates, its sub numbers absolutely tanked and the game went free to play.
In contrast, WoW
constantly updates, and that's why its subs are so (relatively) reliable. A new patch, raid dungeon, and expansion are
always in the works at all times, and they (usually) don't take forever to come out.
Any MMO that copies WoW, has better graphics, and has the same or better quantity and quality of WoW's endgame raids will instantly kill WoW and take all of its subs--
but it hasn't happened yet. No other MMO has even come close, and most don't even try. Rift was the only game that tried to copy the formula, and it worked for a time, but their dev team was not big enough to compete with the quantity of content that Blizz can create.
There are a lot of people out there who constantly accuse MMOs of copying the WoW formula, but this just isn't the case. If MMOs were to copy WoW, they would be infinitely more successful than they currently are. To date, we still have MMOs coming out--such as SWTOR and FF14--that
can't even get ability responsiveness correct. That is not copying WoW.