Why is halochah more stringent for a helpless baby than for an adult? How is a baby-who-needs-help-to-open-the-womb different from someone who needs help with doing brit milah? or writing mezuzah? or making tzitzit? or teaching Torah to his children? And why should the parents of a C-section baby-who-needed-help-to-be-born be denied the nachas and merit of doing a mitzvah d’oraisa like pidyon haben? and even brit milah on the 8th day?
Shalom, Thank you for your question. I am not certain that I have fully understood you, and I beg your forgiveness if my answer is unsatisfying as a result. The mitzvah of pidyon ha'ben (redeeming the firstborn) is not on the baby (who is exempt from all mitzvot), but rather on the father. He is commanded to redeem the first born son who "opens the womb" of his wife. If the baby did not in fact open the womb, then there is no command to redeem the baby. The oral Torah explains to us the definition of "opening the womb", and excludes a c-section baby. As such, there is no mitzvah. The parents are not "denied the nachas" – rather, just as if the first born was a girl, or the parents are Cohanim, or it is a second or third child etc, there is no mitzvah. When Hashem does not command us to perform the act, we fulfill the G-dly Will by just that – not performing the act. That is what the word "mitzvah" means. A command. When there is no command, then there is no mitzvah. (Though there is still tremendous nachas at the birth of every baby!) The other cases you mentioned are not similar at all. The command is on the person to put up a mezuzah etc. The mitzvah is not defined by "those-who-do-not-need-help". It is defined by the doorway. Once the person is obligated in the command, then if they need help to perform it, they may enlist the help needed. A better parallel example perhaps would be "why does someone who lives in a tent with no doorframes not get the nachas of putting up a mezuzah?", or "why does someone with no four cornered garment not get the nachas of wearing tzitzit?". The answer here is simple – there is no mitzvah! I hope this answer is of help. Blessings.