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Why can't you just have someone invest for you and split the profits (and losses) with him?
The 2 and 20 rule is a premium arrangement that hedge funds offer and venture capital funds offer, and they also offer different variations of it. The 2 is the management fee as percent of assets under management, the 20 is the profit cut, which they only get if they are profitable. There are 0/20, 1/15, and many variations. You're assuming that nobody offers this arrangement because it isn't offered to you, but that's because nobody offers it to people that aren't wealthy enough to legally qualify for their fund. When you park 6 or 7 figure amounts in bank accounts, they'll send your information out to the funds that operate the way you wish they operated.