And most hardcore raiders have a few alts and paying for enchants for your gear (500g for mongoose everytime you get a new weapon as a rogue), paying repair bills, paying for consumables, etc all adds up. Now imagine having to go farm daily quests once more to get your alts epic flying mounts...

I don't think you can make a case for hardcore or casual being more likely to buy gold. I think it is different from person to person, some people will buy gold, some won't.

Your point stands about the 5000g cost of epic flyers being the main source for gold buying.