It would be controversial, depending on what country of jurisdiction was supposed to be applied on. Meeting a ceiling of maximum wager for high rollers would be translated to setting a ceiling to the earnings and profits of casinos. Time it would be more favourable in terms of morality for gamblers to be fully aware and not drunk while gambling, to make them understand the degree of the risks and the potential consequences on their lives.
Ironically, it would be like setting a ceiling on the amount of beers or the numbers of cigarettes people is supposed to consume, at least, in countries like the United States, those laws could be perceived as an direct attack on the basic rights of each citizens.
Though, I am not against increasing awareness on problem gambling.
Not sure about limiting high rollers but I have surely seen a maximum cap on betting on one of the sites.
One point to notice is that Watanabe was losing around $5 million dollars a day and so it took him so many months to lose what he had.
So it's not like he wasn't aware of what he was doing. Even if he was drunk while gambling, he would have noticed that he has spent millions in gambling.
He did have the chance to stop himself but didn't. May be was addicted to gambling or he deliberately wanted to lose it all.