FYI, chainlink will solve this problem. They are working on a decentralized oracle solution. You should really read the white paper. Your concern is addressed b

FYI, chainlink will solve this problem. They are working on a decentralized oracle solution. You should really read the white paper. Your concern is addressed by having set a collateral demand for nodes participating. So for example the stake is x you want to have an even distribution of x/n nodes to actually insure your off-chain data. If they deliver, smart contracts will actually take off because they can reliably use external data. Right now, there is no way to do so.