^
These WoW clone MMOs last what 2-3 years before going f2p and dying?  So lets average that to 2.5 years (30 months) $15x30 = $450 + say 2 $50 expansion.. $550 per player who stick around start to end?
 
Now lets look at the games who have kept players around under a sub model for over 12-15 years EQ and XI.  (well focus on EQ because without looking XI's content was a bit more complicated mixing expansions and smaller paid content patches)
EQ = original + 21 expansions.
per player
expansions value = over $1,000 
sub value = $2,900 
Does your average WoW clone bring in a huge influx the 1st couple months that likely make up for the difference?  Sure but lets then realize the cost of developing a new MMO vs adding an expansion to an existing one.
Then lets realize that apparently EQ/Xi were good enough to keep a lot of the SAME people engaged for YEARS.  Where even the titan of these modern MMOs WoW has a almost 70% attrition rate after the 1st 20 months of playing (since I know someone will bitch below are the numbers).  WoW stays afloat by being the equivalent of cigarettes/beer/weed to the MMO world.  Everyone tries it then goes looking for something better.
40% quit after a year which goes up to ~70% at 600 days (about 20 months)
http://netgames2008.cs.wpi.edu/slides/chen-wow.pdf (scroll down to the sub time graph)
 
.... after 8 years.  I never played it myself but that is longer than most MMOs last these days.  We could go through a very very long list of MMOs that actually failed before we ever even talk about SWG shutting down.  And sustained a sub model.
Also since you either don't know or like to ignore important details.  It failed AFTER being completely overhauled due to a disagreement within the business (as I understand without looking it up Lucas Arts and SOE had disagreement)