All manufacturers face the same design decision - once the battery life declines, do you cut the CPU speed and keep the battery life, or do you keep the performance but the phone now dies halfway through the day. I'd say Apple took the correct approach here - 100% of your customers will notice their devices dying much earlier than expected, most customers won't notice the phone getting a bit slower. My mum never ever would for example, she only uses hers for making phone calls. If the phone app takes 2 seconds longer to open she doesn't care, she cares about having a phone that works all day long though.