The thing is, the fact that your code relies on someone else's, and that their code can change out from under you is exactly why you need separation in your tests and modularity in your code. Using the system I am describing, you are still working with 'real world' data - you are just asserting it in advance (since the real world inputs correlated with the inputs of your integration suite via the edge tests) and ring fencing your endpoints.

I've actually been part of a software team driven wild by unknown changes to infrastructure endpoints, causing failure of the integration test suite. And then by the brittleness of an integration suite too tightly coupled to inputs.

I've also worked with test suites that failed when run after business hours, but only during Atlantic DST, for the same reasons. :|