Without need to look the studies and statistics, I think everyone know if the wealth gap between the poor and middle class with upper class are extremely far.
The poor and middle class aren't trying hard to become rich, because their mind are mostly want to earn a lot money via instant which actually impossible! majority of the people this group are spent their money to buy branded stuffs in order to get respected by many peoples. Do you know which one is selling the branded stuffs? it's the upper class!
The upper class already have a lot business, they're also make a lot money from stupid poor and middle class, so it make them can expand their business become bigger and bigger.
10000 % I was agree with your statement there is always gap between upper class and lower class in fact this might happen when human exist centuries ago.
and in my opinion and like other have said this might can't be fixed the upper class make business and make them getting more rich and the lower class buy it