My primary point is that "disparity" has diddly squat to do with "feudalism", or well-being in general. It's a red herring.
Let me put it this way: if disparity is bad, what kind of society would have "zero disparity"?
Here are a few possibilities:
1) everyone has $10,000. Period. How do they buy anything? What do they buy?
2) everyone has the 10,000 rolls of toilet paper. I'll give you 10 rolls of toilet paper if you give me 11. No? Why not?!
3) everyone has $10,000 of something, and now they get to figure out how to redistribute it ... at which point the disparity will disappear ...
A healthy economy comprised of a largely wealthy people will necessarily have a fairly significant "disparity." Otherwise, there is no reason for people to trade.
A feudal system might have what is qualitatively (not quantitatively) described as a huge disparity, but the feudal lords' wealth is very much limited by the serfs' poverty.
There is only one reason that "disparity" can be wrong: that the minority of wealthy people took the money from everyone else. This is the cornerstone of Marxist philosophy. In reality, if a society is wealthy, then the mechanism of "the rich get their money from the poor" is a ridiculous hypothesis, because no one can (or would bother) to create (much) wealth in such a scenario. Only in a free society which respects property rights can private individuals (you know, people not necessarily running the government) become "obscenely" wealthy.