Doodledog
Children have always pushed boundaries for a reaction. People who hit their children rarely did it once, and teachers caned the same children regularly.
Small children don't necessarily understand why they are behaving as they do - they are tired, or scared, or jealous, or overwhelmed, so they lash out, or cry or whatever. Teaching them to deal with the emotions, rather than blocking the reactions to them is more likely to result in well-balanced teenagers and adults.
Gentle parenting is harder work than threatening or violent parenting. It needs patience and persistence from the parents, who may be tired, stressed and sick of others looking askance at them and their children. Parents who manage to stick to it are likely to be invested in their children, so I don't think we need to worry about their futures.
I am married to an only child. Throughout his adult life he has had difficulty dealing with negative or aggressive (not violent) behaviour from other people. I have always put that down to his being an only child in a very peaceable family. Until he left home, he never really faced aggression or dislike or hate from people, except in situations where he could retreat home.
This inability to cope with anger, dislike and all negativity in other people has caused him real problems in adult life.