One specific thing I think FP distributed-system frameworks could bring is instantaneous, deterministic testing. e.g. I have N microservices, they communicate

One specific thing I think FP distributed-system frameworks could bring is instantaneous, deterministic testing. e.g. I have N microservices, they communicate via IO (which means slow/brittle tests) but the framework abstracts over that, so I can run the whole system test suite under a second, because all IO (messaging, DB, concurrency) can be replaced with an in-memory implementation. Interestingly, one could also inject faults to those IO mocks, and test how the system respond to that. I believe FP is a prerequisite for meeting those goals, because otherwise having impure IO all over the place is hard (if not impossible) to mock out.