I couldn't find the statement online after an admittedly cursory search, but I seem to recall somewhere that even Blizzard has stated that without revenue from their cash shop, they would have been forced to raise subscription fees to combat increased technology costs and inflated developer salaries. Customer response to that was overwhelmingly negative, so they took the "optional but costs more at the individual level" approach instead.

Basically, development costs for MMOs have gone up substantially in the past ten years and yet player willingness to spend more than $15 a month has remained very inelastic. For what it's worth, I'd rather pay a higher sub fee, but given the omnipresence of quick-and-dirty freemium games that don't require an initial investment to start up, it seems I'm in the minority.