Yes, the Gambinos made a lot of money under Castellano. But, according to Kurins and O'Brien in "Boss of Bosses," Paul lacked the one quality absolutely vital to a "good" Don--the respect of his people. He wasn't respected because:
--He almost never went into the streets to meet with his people. They had to come to his home in Staten Island--and not many were invited.
--He cut deals with other families that his people felt were giving away their business.
--He involved himself in petty details and small amounts of money that should have been left to underlings.
--He didn't show up at Neil Dellacroce's wake or funeral, which members of Neal's crew took as a sign of disrespect.
--He carried on openly with his Colombian housemaid, Gloria Olarte, under the same roof where his wife lived.
--After the Commission case was filed, other Dons believed that he'd turn rat rather than spend the rest of his life in prison, away from Gloria.
And, most important:
--He was stingy with money, refusing to share generously with underlings. If you want to look for a source of Mob violence, just follow the money.