I have a counter-example, which has nothing to do with spanking. When our children were very small and we’d take them to a restaurant, we’d always a) take them something to do, b) bring something they could eat if the service was slow (asking the staff permission first), and c) bring a dustpan and handbroom in the back of the stroller, to clean up the floor afterward. They were taught to say “please” and “thank you” and to stay at the table for the duration of the meals at home, so it was no shock when it was required at restaurants, too. They were given to know that if they acted up in a restaurant, they would be leaving their food behind and escorted to the car by a parent who would not be engaging in enjoyable conversation when they got there (their best hope would be no conversation at all!

), while their behaving family members would be enjoying the rest of their meal at their leisure.
As you can imagine, they got a lot of positive feedback for their behavior by restaurant staff. As a result, they saw themselves as people who act a certain way, as patrons that restaurants actually wanted. They themselves were appalled by peers who acted like animals who needed to be on a leash. They could be allowed to supervise themselves in restaurants by the time they were in middle school.
My point is that punishment can never take the place of a self-concept that says, “I control myself so as to be thoughtful of others, and other people appreciate that about me.” I agree with you: Parents who do not teach that to a child leave their child impoverished.
I don’t think that the biggest mistake modern parents make is that they don’t spank. I think it is that they think a terrier can be trained to behave and a child can’t. It is that they think their child has some right to be liked no matter how he treats others, that because their child has some feelings of empathy for others that empathy alone will teach him the manners expected in our culture. IOW, people expect their children to be mind-readers, hoping blindly that they’ll behave but never giving them firm training regarding what “to behave” even means. “You’re kind of making a mess” and that you have room to complain when you feel bad because your own bad behavior is pointed out to you? Good grief! That is so short of the truth as to be a lie! Why would you lie to your kids like that?