I don't think that's true. The rich will definitely earn more money than people like us just because they are also investing money much larger than ours. But if their investment fails, then they also lose a lot of money compared to us. I believe that investing is not for everyone, and definitely not just for rich people. You can still diversify your investments if you don't have that much money. As for me, I invest mostly in BTC, ETH, a small bag of alts, and shares of
a stock. It doesn't matter how much you invest, its how you invest it that matters.
You are right, investing can be for anyone but what I disagree with your statement is that you said that it doesn't matter how much you invest because I believe that the amount that you are going to put in the investment is going to help you in the long run, if you invest 10 USD rather than 1 USD then the difference of 9 USD is going to matter when the investment pays off. Yes diversification is good but that is if you already have a strong base, concentrate on one or two investments before diversifying your portfolio.