There are people with genetic defects that prevent feeling pain. These people constantly injure themselves.
As soon as the baby and placenta are out, it's too late to avoid injury during childbirth. It is no longer possible to let pain be guidance for minimization of injury. The chance was missed.
Have you seen a childbirth, and one without an epidural? There's so much pain that it's not going to be guiding anything. Physicians monitor the status of the infant via fetal heart rate monitoring and monitor the progression of the delivery.
I have seen more than a childbirth. I have 12 kids.
The most miserable and painful birth was in a hospital. That birth featured pain medication, fetal heart rate monitoring, beeping machines, and a prison-like situation with doors that locked us in. My wife was in pain for months afterward. We learned our lesson and didn't do another hospital birth.
The other 11 births were definitely not "so much pain", but pain did guide my wife to be careful. She was always up and about right afterward, without pain. One time she walked around town doing errands the next day.
Pain definitely does have a purpose. It alerts you to things you may need to know about. Once you're alerted, its purpose is mostly lost. And many, many people living with chronic pain can tell you that sometimes the message is worse than the cause. The fact that complete absence of pain is a problem in no way supports your imperative to not get an epidural during childbirth.
I agree that getting an epidural before handling sharp objects is unwise.