The solution seems straightfoward to me. Slow down the phone, and output a message saying 'this iPhone is being slowed down due to an old battery. Replace the b

The solution seems straightfoward to me. Slow down the phone, and output a message saying 'this iPhone is being slowed down due to an old battery. Replace the battery to restore performance'. Then have a warning sign on the battery screen with a similar explanation. My wife's iPhone got throttled, and it felt really bad to use. This wasn't some small hit to performance that you can just do without informing the customer.