What Return on Investment are you speaking of? The only investment they need to consider is having subscriptions, or not having subscriptions. Thats all that matters in a Pay to Play MMO.
What people seem to be neglecting with their argument of "spending millions of dollars on developing content for only 1% of the player base" is that it worked for almost 7 years of the game's life to increase subs. By having exclusive content. Its only been in the shift in philosophy that content be inclusive that WoW has lost subscriptions. I don't think you can call that coincidence.