Re: This would be easily fixed
I don't think you are getting it, your phone won't work consistently if you turn it off.
This isn't about extending battery charge via CPU throttling via normal power management. That's just a side effect, not the purpose.
This is about system stability. The older batteries can't cope with peak demand anymore, therefore the CPU is throttled in order to stop the phone from crashing/rebooting when demand is too much, due to voltage instability.
If you switched this off, you'll end up with a phone randomly crashing and rebooting whenever it was pushed a bit harder. Which is the opposite of what you seemed to be asking for with regards to consistently working.
PS: I dislike how Apple have handled this. But technically (I'm an electronics engineer) I can see why it was done.