Try this thought experiment. Suppose each of us lived on our own desert island, like Robinson Crusoe, with identical resources and skills – so we’re all perfectly equal – and get our food in the form of fish from the teeming oceans (there is no scarcity of fish). Then suppose one of us works out a way to fish better, so inequality increases. Is everyone else somehow worse off? Clearly the answer is that everyone else is not worse off unless the better fisherman makes fish scarcer for them. The one person’s riches do not come at others’ expense.
Obviously this is a rather abstract thought experiment, but it points at something simple and important: almost all inequality in developed economies does not arise by the wealth of almost anyone else declining. (That does happen in less socially and politically developed societies, in which wealth arises from political control of resources or access to corruption.) In modern developed economies inequality arises when someone – a Gates or Zuckerberg or Cowell or Ronaldo or Rowling or just an ordinary businessman or professional – finds some way (some skill or invention or investment) that adds considerable value, and that value is not then shared equally.