So, why are we all screwed?
As I mentioned before, Blizzard has done an exemplary job of creating an environment in which gold is introduced to the economy via grinding and questing, but removed via vendors, mounts, and the Auction House fees.
Throw gold farmers into the mix and you end up with a bunch of Level 70s who can dole out damage and kill mobs like any other 70 worth their salt (well, maybe not the furry warriors). The thing that differentiates them from other 70s is, they don't give a piece of vendor trash about getting an epic flyer or gearing up in instances. They exist solely to grind for hours on end and convert their gold to money IRL.
The overall effect, if you follow me, is a bit like what happens when a band of counterfeiters begin to slip a bunch of phony $100 bills into circulation. There's suddenly a huge influx of money to the economy chasing a supply of goods that remains essentially the same. (Supply and demand, of course, is the principle that makes even the crappiest BoE epic sell for no less than 500g on a bad day in the trade channel.)
What does this cause? Well, in a word: inflation.
And who will suffer? It won't be the 70s in full tier 6; it won't even be the 70s who are struggling with an assorted mishmash of quest rewards and greens. It'll be the newbies, many of whom still think of one gold as being a large quantity of in-game money, who will suddenly balk at 10g stacks of copper ore, 50s Minor Healing Potions and ungodly expensive low-level greens. When a typical mob drops a handful of copper and quests reward sub-gold quantities of money, players will either beg or let their subscriptions expire. Sure, they could turn to gold farmers to alleviate this inflation, but of course that'll only exacerbate the problem.
Yet there is a glimmering, glowing, radiant, brilliant, even prismatic glimmer of hope for the game...