> If 90% of the population had a (sustainable) lifestyle of current dollar millionaires, and the remaining 10% had a (sustainable) lifestyle of dollar billionaires, there would be lots of inequality, but it wouldn't be problematic.

You are looking at this from a world where that isn’t the case. Money would be worth less in that scenario, and that 10% owns 99% of the wealth, which is still going to cause problems.