Yes, people think money is something that can't buy happiness but if we think of people who don't have money and struggling everyday for a single penny in order to get food then we will get the answer... If they have money they won't need to struggle that hard mind you sometimes it's hard to get happiness like some people depends on love and find happiness with their partners so I am on the fence neither on the negative side nor the positive one
Happiness is a not a thing but its an expression or emotion and I think that was the person thinks when he said that quote about money cant buy happiness although for we us we think that is wrong because our happiness comes out when we have a money or when we use our money to buy a certain things but your assumption is wrong about people that have a money will not work hard anymore.
We often heard stories about billionaires that still work hard and follow a strict routine every single day. For the attitude, it depends on the person and not the money that they have. There are poor but still nice but there are rich that easily get irritated and angry at all time.