I can sorta understand their perspective (which is not to say I sympathise).
The jump in living cost from middle class millionaire to the level where you "need" an income from investments of USD250-500K a year to maintain your accustomed lifestyle - luxury cars, boat, holiday home(s), country club, children in private schools, overseas holidays, mistress etc is significant - like tax rates, prices are progressive and advance more rapidly at the luxury end of the scale. As an example, a round trip ticket HK-NY-HK costs 5-6 times as much in business class as it does in economy. Inflation has also been historically higher for luxury goods and services than for the economy in general (see the Forbes Cost of Living Well Index).
Then there's the comparisons. USD15 million is rich, but your chances of rubbing shoulders with people who are (or appear to be) significantly richer would (I expect) become greater the more money you have. Comparative wealth matters to people's sense of well being. Expectations go up and its hard to bring them down again.
Lastly, we're living at a time when populist calls to tax the rich more are being answered and it's widely believed that countries which have racked up monumental debts spending beyond their means will have to inflate their way out of the mess (as well as continue to raise taxes). It's little wonder that people are more than a little bit paranoid about maintaining their standard of living.
That said, I'm sure it would be a nice problem to have and I'd like to think that if I had that kind of money I would keep a slightly healthier perspective.
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