I think caring parents, regardless of whether they regret having children, recognize it will make those children feel like shit to hear you say wish you hadn't had them. I hope I am not interpreting incorrectly what you are saying, but I think it's not merely the threat of abuse from others if they say they regret having kids. Legitimate care for their children even into adulthood may be preventing parents from saying this. That will make the taboo hard to break, if it were even desirable to do so, which I don't think it is. The question is important, but breaking that taboo not so much.