I Became the Contract Daughter of the soon-to-be-ruined Family - Chapter 4

I don’t understand this guy. It would have been better to just adopt a kid and lie to them that they are the lost daughter. The kid would have always acted the correct way. And he could have kept the secret that they weren’t to themselves. If his wife finds out she will be devastated even worse than if their original daughter was dead. After that she would have lost the original daughter and filled alienated to from the one that she thought was hers.