You showed why it is worth it - it also avoids copying. Mutation is more efficient in every way, except cognitive overhead. So by preventing _sharing_ of mutable values, you get the best of both worlds.

This is the same model that Rust takes too. It doesn't eliminate mutability, only controls it. The argument that "performance doesn't matter anymore because computers are so fast" is a bad one. Efficiency is efficiency, and immutability will always be less efficient.

It's certainly fair to balance/decide between performance and local reasoning. But one should be clear then that they give up one for the other and not claim that both is possible. Because from my understanding from what you said, local reasoning isn't possible anymore, even though reasoning is still easier compared to when you don't have struct semantics. But that's still two different things.

You still have local reasoning with struct semantics. You are not giving that up in exchange for performance, you get both.

  func newStructValue(s: Struct) -> Struct {
    s.value = 5

    return s
  }

The reasoning for this block of code is totally local to the function body. Because the only reference to `s` is in the body of the function - it cannot be more local than that.