Okay, lets say we have an obscure newfangled currency we invented, of which we have shitloads on hand, but for some reason common Earthlings don't seem to think our wonderful newfangled currency is actually worth much, if worth anything at all.
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OK, apologies if I strawmanned your explanation but here's what it sounds like to me. You've got Pirate backers with assloads of BTC. They're giving BTC to pirate (let's say half, bs number) which he uses for the interest. The backer's gamble is that the popularity of Pirate's scheme will drive up the value of the BTCs they didn't give to Pirate, putting them ahead and growing bitcoin.
This still doesn't beat out starting/investing in a business: if you took half of your BTC and started a business, not only would the half not invested grow in value as the BTC economy grew but the business could pay you back your initial investment through dividends and equity sales.
It also doesn't explain why Pirate's having a bitch of a time paying people back. Your model has Pirate's backers and Pirate never touch any USD so why is he taking so long to repay? Pirate himself has said on IRC that he doesn't keep BTC around.