Sorry but I disagree with your views. If you Novak broke his racquet a warning would be sufficient. If he hit the ball and it didn't hit anyone a warning maybe sufficient. But as it resulted in injuring a person the punishment was absolutely justified. Just imagine that being a ballboy, or it causing long term damage or even death. Everyone knows it was unintentional, but it could be serious for the person involved.
The rules which is in place is absolutely right one because it puts the health and safety of its stakeholders above a game. If it was the other way around it would be just disgusting.
To be honest Novak understood the decision, he was disappointed but he knew the decision was inevitable.