[fastball]

  There is only so much land, food, and materials to go around.

True(ish). False. False.

I say "ish" for the first one because hopefully humanity will grow beyond Earth in the near future.

Generally, all of the above are false unless you are talking about a population which never stops growing, and in that case that would be the problem. Not wealth inequality.

[int_19h]

If you have a population that did stop growing, but in which class divisions exist, and the upper class keeps taking larger and larger share of overall wealth generated by the society, you're going to have a similar problem.