If Ethereum or something like it were to succeed, it would end up taking the form of low-level plumbing for financial products or other stuff which will simply

If Ethereum or something like it were to succeed, it would end up taking the form of low-level plumbing for financial products or other stuff which will simply say: "built on the Ethereum trustless/blockchain system". And as others here point out, there would be an ecosystem with companies which would audit and rate smart contracts for trustworthiness by an end-user. That in itself might end up being another application of Ethereum. Again, emphasis on the aforementioned if.