How would the back find someone trustworthy people to loan to? I suppose they could have some USD in escrow and have a way to force them out of their position if they are reaching their escrow limit. But all in all, it requires a lot more infrastructure than we have at the moment.
you don't have to trust anyone, all you really need is a liquid market, and quick automatic liquidation of positions.
one exchange already announced to implement it. at high rates though...and they dont have liquidity yet so you probably have to expect very high margin rates.