All you have to do is make sure the giant losses occur just 1 year out of N. It doesn't matter that the one year's losses wipe out the other N-1 year gains; you've already collected your fees and quite possibly retired, and there's no claw-back of your cut.
A reasonable aim of financial technology is to ameliorate the risks that could obscure or destroy truly productive activities. But the same tools can easily be used to do the opposite, construct net-wealth-destroying strategies which enrich only money managers.
To cook up such a strategy purposefully is a crime, and the classic ponzi scheme is the blatant extreme case. But many other strategies based on collecting fees during the upswing of an unsustainable bubble, while being insulated from the downswing, are close cousins of the full ponzi. That the loot is laundered through ostensibly legitimate investments (like mortgage securities this time around) is a convenient consensus cover story; it helps even the perpetrators believe they are providing valuable investment services rather than just a scam.
This view doesn't indict all investment activity/funds as being similar to ponzi schemes. But an awful lot of this most recent financial-services boom appears to have been like a ponzi skim.
(Despite the truth I see in this column, I would not trust Krugman's suggested solutions, which are likely to be heavy on new regulation. Ultimately capital has to learn to protect itself, with novel compensation schemes that disincentivize the 'year N explosion' pseudo-ponzi strategies.)