This statement is enormously naive. Bill's money has to come from somewhere so the "everything else remained the same" is just not possible...
Unfortunately if people are getting much richer than other people it is necessarily at other people's expense - a reasonable approximation is to think of money like energy, which can never be created or destroyed (its not quite true) - Bill doesn't make his pile of money out of nothing leaving everything unchanged, the money he makes comes from other people's piles of money - these need not be poorer people, in fact I bet most of them are big businesses, but then they don't get their money from nothing either, and their budget decisions on how much to spend on MS software will be balanced alongside salaries etc... at any rate, that money moves around and naively, if you made too much you would quite literally be making everyone else poor. I don't have access to huge amounts of data needed to workout how this scales in reality and if Bill is anywhere near the point where he is almost certainly making everyone poorer, but it is a trivial consequence of the nature of money.
This is the nature of capitalism - if every one does what is in their best interest, some talented/lucky/greedy people will end up much better off with zero incentive to improve the situation for anyone else. Thankfully capitalism is merely an ideal and nobody follows it that blindly - Bill at least tries to give some of his money away to good causes lately...