That was not to get you to buy another phone, in fact the opposite. It was to keep your phone functional even though it had a worn out battery.

In phones there is this concept called a ‘race to idle’. Basically, you want your phone to do nothing, because doing nothing uses very little energy. So when you do something on your phone, the goal is to do it as quickly as possible so it can go back to doing nothing and save battery. Your phone will be in this low-power idle state 99.999% of the time. You still want your phone to be responsive though, when you click on something you want it to respond without delay.

Now, iOS is really aggressive in this, it ramps up the CPU speed really fast. As a result, the power draw of the CPU goes from almost nothing to a high power draw very quickly. This causes problems with old batteries. As a battery ages it not only loses capacity, but it also becomes slower to respond to changes in power draw. If the CPU needs a lot of power quickly and the battery can’t keep up you get a brownout (drop in voltage) and the phone basically crashes and reboots.

So what Apple has done is that when iOS detects this happening (i.e. a crash due to the battery being unable to keep up), it will ramp up the CPU a little slower. Or to use a car analogy: they don’t change the top speed, but are less aggressive on the gas so it takes a little longer to get to that top speed. If you replace the battery it goes back to the original behavior.

This is basically a good thing, the alternative is that your phone keeps crashing. Where they screwed up is that they failed to inform users of this.