This article outlines the basic problem. If you want smart contracts that do anything off chain, there have to be connections to trusted services that provide information and take actions. If you have trusted services available, you may not need a blockchain.
The article points out that you can't construct an ordinary loan on chain, because you have no way to enforce paying it back short of tying up the loaned funds tor the duration of the loan. Useful credit fundamentally requires some way of making debtors pay up later. It's possible to construct various speculative financial products entirely on chain, and that's been done, but it's mostly useful for gambling, broadly defined.
The article points out that you can't construct an ordinary loan on chain, because you have no way to enforce paying it back short of tying up the loaned funds tor the duration of the loan. Useful credit fundamentally requires some way of making debtors pay up later. It's possible to construct various speculative financial products entirely on chain, and that's been done, but it's mostly useful for gambling, broadly defined.