I agree with the general idea that people shouldn't be automatically assumed to be trustworthy, but there's something very fundamental you seem to be missing, and I'm happy for the opportunity to point it out.
Foolproof schemes to make high interest at low risk are the financial world's perpetual motion machines. They do not exist.
Perpetual motion machines may not be scams; there are misguided people who really do believe that they've violated the laws of physics and will prove all of those pesky scientists wrong. I'm sure there are people who truly believe that they've invented foolproof methods of investing lots of money with no risk, too, but I'm equally sure they're delusional.
If someone is taking deposits and offering high interest with low risk,
it does not mean he thinks whatever he is doing with the money is low-risk. It means he is willing to absorb the risk, and commit his personal illiquid assets to pay it back if the risk doesn't pan out.
Let's take a simple example. Someone is very sure that the BTC price is going to increase and wants to buy $10K worth of bitcoins. But he doesn't have $10K and can't borrow it with traditional means. He does have a $20K car which he'd like to keep if possible (and which the bank won't accept as collateral), but he can trade it for a $10K car should the worst happen. He expects to sufficiently profit from buying to be willing to borrow at high interest; and he commits to repay by trading his car if he loses.
This whole arrangement is very risky for the borrower (whether he realizes it or not); but if the borrower is trustworthy, it is not risky for the lender.
In practice there are multiple ways to generate high ROI with various risk levels, such as currency speculation, arbitrage, mining, starting new businesses in the emerging Bitcoin economy, short-term loans, or investing with other people who are better equipped to generate ROI than you. By diversifying among various channels the overall risk can be lowered even if every single investment is risky; and by having valuable traditional illiquid assets there is a way to honor one's obligations even if the risk assessments went horribly wrong. In this case a smart investor can profit by accepting high-interest loans with no risk for the lender.
I'd like to especially point out short-term loans as one of the primary revenue streams of some of the big borrowers; many people are willing to pay high interest for such loans. I don't know why they are, but the fact of the matter is that they are. Once again it is the borrower's role to assess and absorb the (relatively high) default risk of these individual loans. This isn't new with Bitcoin, also; p2p loans with 1% daily interest have existed for a while in the traditional economy.
To summarize, I'm not saying any particular borrower is or is not a scammer. I'm saying that given the diversity of investment opportunities, there is nothing fundamentally impossible about high-interest loans with no risk
for the lender. The only catch is finding a borrower who can be trusted to be both honest and responsible with his liabilities; and of course, by the time it becomes easy to find one, the supply of time-money will be high enough that his offered rates will not be as high.