Because you must statically declare dependency on an effect, it's opt-in. To be clear, you still pay indirection cost: when you do opt in you have to hope the

Because you must statically declare dependency on an effect, it's opt-in. To be clear, you still pay indirection cost: when you do opt in you have to hope the upstairs implementation is compliant to the contract. But that does also apply to interfaces/typeclasses.