I don't think it's to do with disposable income vs whether you have money tied up in property. You still have the money, irrespective of lifestyle. You could sell the property, change things, I don't think you'd 'become' rich - you still have the same amount of money, it's just not tied up in property and you've changed your lifestyle.
I'd say top 10% of USA is rich.
Worldwide, most (over 50%) of people in the USA would be considered rich - nice home (relatively), TV, mobile phones, shower, indoor toilet, carpets, satellite channels, wardrobe full of clothes, fridge full of food, able to eat out once a month or more.
This is the correct answer. And you all should travel more in order to witness what is it like to live within your means for the rest of world. You do not have to go to Nepal, or Sudan to see the lowest level of poverty, but living comfortably, also in first-world countries, is to have a lot less than you Americans have. It does not come out of thin air, the goods have to produced somewhere, the environment gets polluted somewhere, the mundane and tedious jobs have to be done by someone, so that you can live in your self-imposed image of comfort. Well, don't be shocked if that perception changes drastically within the next decade. And it will, everywhere, not only in the U.S.
The rich will have a very different meaning by then. Usually the changes in thinking, trends and views come from the most developed countries and spread to the rest of the world within 1-10 years. It has been the case of the U.S in the last 80 years, however, this time the U.S. would be the last one to catch on (good for you, Americans, bad for the rest), but it's inevitable. The more we will resist this change, the more it will hit us and catch us by surprise. Would it be war, climatic, or other catastrophe, civil war? Who knows, but it will happen soon. The stocks, equity, 401K, ROTH, paid-for houses, disposable income - all gone. The real asset will be drinkable water, shelter and some stockpiled non perishable food, maybe a weapon if you are skilled to use it.
Enjoy the company of loved ones, and the luxury that you don't have to fear for their and your life every single moment of the day. It's not given, and it won't be within 10 years.