So the choice is, your old phone either just dies or it becomes sluggish. I'd rather it become slow and I can still use it than the battery just die.
There are more options than that - like
1. Notifying you the battery is faulty so you can replace it
2. Decreasing the time it can be in normal speed before it has to go into low power mode
3. Not slowing it to the point of 1/3 the purchased speed (600Mhz vs 1800Mhz).
4. Giving you the option to do none of the above and work as purchased before the OS hack that did this was applied!
Good for you - I hope its recycled responsibly.I buy a new phone every year so this doesn't affect me.