If we think it's okay to spank and overpower a little child, we did NOT turn out fine. If we don't/didn't hold grudges against our parents for hitting us, it's because we formed insecure attachment patterns, were conditioned to believe that we deserved it, that our feelings didn't matter, that we wouldn't be good people if not fearful, that it was okay for someone bigger to use their might against us and violate our body autonomy to control us. All these are indicators of psychological abuse.
Also, consider the following questions: I'm sure you have heard of the saying "Might is not right". Is it fair to use our mightier position as parents to subdue a small, vulnerable child who can't fend for herself? Would you hit your spouse/parents when they don't listen to you? Of course not! Why? Because they'd probably hit you back, push you away or make sure you suffer consequences. Your child on the other hand can't defend herself in these ways, and is wholly dependent on you for survival. Is it fair then to take advantage of her weakness?
Kids are whole people, and they deserve to be treated the way we'd like to be treated. Would we respond better to people who hit us, or those that take the time to relate to us?
Kids do the best they can with the skills they have. Is it fair to punish a child for acting in a way she's supposed to, or for behavior outside her control? Would you rather your child fake compliance out of fear, or feel safe to show you how he truly feels? How would your child learn to stand up against unfair treatment when he's used to being treated unfairly at home?
If your child is conditioned to only respond to physical punishment, how would he learn to do the right thing when you're not looking? Would you rather your child do the right thing out of responsibility or out of fear?
What would you do when your child becomes a teenager and you can't spank him anymore? How'd you get his cooperation then? The only lasting influence you will always have is your relationship. We all listen better to those we feel connected to, who take the time to understand and relate to us.
As parents, we are responsible for modeling the behavior we want to see in our kids. We can't teach kids to behave and refrain from yelling, hitting, aggression when we don't. You can't teach your child respect by being disrespectful.
Lastly, if there are ways to get your child to cooperate without using harsh methods of discipline, why wouldn't you try them?
Authoritarian methods tend to assume that humans are awful, terrible people if not hurt and driven by fear. In reality, good behavior, kindness and compassion are our DEFAULT. During the times we aren't our default, there are either unmet needs or big feelings in the way. In Gentle Parenting, we remove these 'obstacles' so a child can return to his good thinking again.
Kids don't choose to be ill-mannered, or disrespectful. They lack maturity, experience and perspective and cannot act like adults. The frontal lobes of the brain responsible for self-control and emotional regulation are "under construction" in little ones, and they need our help much of the time to listen and be cooperative. We adults, on the other hand have a fully-developed prefrontal cortex and are completely in control of our behavior. If we hit, we're the ones who're ill-mannered and disrespectful.