let (model', cmd') = state.update msg

In order to write that you have to pin a reference to the state. But state is something that has to flow. Sure you can pin it to a ref, make it mutable and change the state but then nothing prevents you to change the state arbitrarily e.g. from a command. I believe yes the answer is because we want to stay on the FP realm. FP imposes some restrictions to keep our sanity. So not having a reference to the state and making it immutable is one of those restrictions.