Taking gambling as source of income depends on individual and if you don't see it as a major source of income, so I can tell you that I have many friends that depends on gambling to make a living. It all depends on the place you come from if they don't take gambling as serious as you think. So many friends do not have a job of there own so they see gambling as a big survival slot for them to make a living and pat bills and is working for them.
Gambling should never become the major source of income, it's just not healthy. It can be a hobby or an entertainment factor, but depending solely on gambling as an only source of income is not that sustainable and I'm not sure how good it is for your mental health.
I know a few professional poker players that had made a proper career out of it, but I think that there's a difference between a poker player and a slots player.
We should use gambling as a means of entertainment and to get pleasure and forget to make gambling a source of income, let alone make it the main source. It will not work for people who only think about making money but on the contrary, they will lose a lot of money before he knows it.
But it will be different for professional gamblers because they already know how to treat gambling. They can control themselves and make money from gambling, which is very difficult for people who don't have much experience in gambling.