I think a real thing that's not being considered is that real life economies include efforts/capital towards innovation. With mmo economies, the only grind is for existing goods because there's no ability to "invent" items. Thus, all inflation and all grind goes solely towards the purchase of the highest level items, which either have to generated at a rate matching demand, or inflate to infinity because their scarce. 

Sounds weird, but in game economy would have to introduce "invention" as a natural sink for capital to combat inflation whether it be from in game grinding or external pay-to-win players  

 the grinding in MMO's serves the business model - keep the players paying the subscription fees.