Investing in a gambling site is not really recommended in my opinion. Investing your money on gambling sites is very risky, because you might lose your money, and you won't even see it coming. There are dark secrets behind investments on every gambling sites, especially if you are not familiar on a certain gambling site you want to invest your money because you've been carried away because of the hype.
and the word "Investment" has a lot of meaning, by depositing your money in a gambling site just for you to be able to play is already an investment, and also the investment that you will just deposit your money and you'll just passively earn money from it(that I've mentioned earlier) it is like you are investing in some HYIP sites.
I very much agree with your reasoning, but not really your conclusion. I've had a lot of success with bankroll-investing (and margin-lending in fact), but I think the absolute key thing is just being
extremely disciplined with how much you invest. Many people have this (imo, misguided) idea of finding a "great investment" and then put a lot of money into it. But I think it's way better to find "good investments" and put a tiny bit of money into them.
There's definitely a lot of high-fixed costs into any investment (e.g. research) but I think it's really worth while even for nothing but practice. Because yeah, as you alluded to -- almost everyone who wants your money for "investment" just wants it for their own pockets.
(Also another important thing about diversification is continually taking your profits out. Otherwise it might only look well on paper, but it's all fake profits (e.g. a nascent ponzi) and you end up having too much exposure).