http://users.telenet.be/mmodata/Charts/TotalSubs.png
You clearly see that starting from 2009, the total number of sub-based MMOs plateaus at ~22M before declining. In marketing terms, that means the market is saturated. When a market is saturated, you can only attract new players from other games. Also, you need to reduce the churn, which was huge in BC (the devs discussed that back in the days). So, you can no longer afford a lot of players leaving before lvl 10 like it used to be the case. Fact.
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There is not enough hardcore players for that. Unless you insinuate that 4M players who left since Cata are all hardcore, in which case you have problems with stuff other than maths.
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Precisely.
And you can't attract new players when the market is obviously saturated. That's kinda the definition.