Its hard to determine the rate, but it's easy to determine who really are making profit in gambling, and it's non other than the gambling operators.
Aiming to make gambling as a living is a hard job, maybe the hardest kind of job in this world as we know that majority of the gamblers loses over time.
If we have enough capital, maybe the easiest way to make a living in gambling is just to start our own casino or gambling sites.
Its very difficult to make a casino yourself. There is an opportunity to invest in an already functioning gaming site. And there are actually a lot of such sites that want to attract investors for their business. One such site is Mintdice
https://mintdice.com/guide/investing But I am not familiar with the details of investing.
You make a good point and I also support the act of investing in gambling bankroll cause is another way of making passive income. However, the creation of a gambling site is a long process but ones don't need to have huge capital before he can create a because he can just make use of IEO scheme if have what it take.