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Occasionally people here claim that WoW's success is some isolated fluke, and no other game will ever match it. I say that's nonsense. The market certainly still allows for a single MMORPG to have millions of subscribers--because WoW still does.
We see this sort of claim in baseball all the time. There will never be another 300 game winner. There will never be another player who bats .400. And so forth. Until some transcendently talented player comes along and does exactly what we were assured could never happen again. Just this year, we saw a triple crown winner, a rookie nearly win the MVP award, and a knuckleball pitcher with more career losses than wins entering the season win a Cy Young in a landslide. If you could combine CC Sabathia's career up to age 31 with Jaime Moyer's starting at age 33 (taking a year off at age 32, even), you'd have a 400 game winner.
But with MMORPG subscription numbers, the situation is far more volatile than you think, for one reason: marketing. WoW has surely spent more on marketing than any other MMORPG in the last decade, at least in the United States. I wouldn't be the least bit surprised if WoW has spent more on marketing than the next five games added together. If you exclude sites like this one that are specifically dedicated to gaming, I probably see more ads for WoW than for all other MMORPGs combined. Think WoW's subscription numbers have something to do with that?
But it's not so simple as spending a bunch on marketing long-term to buy success. If that worked, everyone would do it. Rather, let's suppose that you're considering a $10 million marketing campaign. You conclude that this would bring you an extra $8 million in revenue. Do you do the marketing campaign? Of course not; it's a huge money loser.
But what if it would bring you an extra $12 million revenue? Then do you run it? Sure: you'd make an extra $2 million that way. And if you ask the same question six months later and reach the same conclusion, then you run another marketing campaign for your game. And then another. And another. Over the course of a game's lifetime, the latter game might well bring in many times the revenue of the former.
And yes, the latter game was "better" (in a commercial sense) than the former. But several times better? A given marketing campaign would only bring in 50% more revenue. And one could make that 50% figure an awful lot smaller and still reach the same conclusion.
Someday there's going to be another game where, as happened in WoW, the company concludes that a big marketing campaign would be profitable. And then another. And another. And another. And keep on doing this for a number of years. And they're going to bring in revenue and player base numbers that invite comparisons to WoW.
It might not even be something that we see coming. Who would have predicted the runaway success of Minecraft? Angry Birds? FarmVille? Sooner or later, MMORPGs are going to see another runaway hit like that. And sooner or later, there's going to be one that bests WoW in its heyday.