I mean, why do you need a fiat in the first and what is that monthly interest rate you are talking about?
It's not only about monthly interest and investments. We all have emergency cases in life and you can't pay for them with bitcoin. What if there is a car accident, you got an injury, need surgery or something emergency happens? We all prefer to have something stable and fiat is the best option in this case. To my mind, it's necessary to have some emergency funds in fiat.
There is no such thing as a more profitable investment in cryptocurrency or fiat, I know very successful people who, with the help of fiat money, get much greater profits than those who deal with crypto.
With cryptocurrency it’s even more complicated, because you can’t count your profit if you only see numbers on the screen, it won’t be your profit until you withdraw it into cash, which you can hold in your hands. I know such successful investors in crypto who owned hundreds of thousands of dollars and ultimately withdrew only a few tens of thousands, all the rest of the profit was eaten up by the bear market.
If one can use fiat wisely and start a very profitable business instead of focusing on passive income, no doubt that will be a good choice but for passive income, crypto is the best option because it's still new, growing rapidly and there is the space for development for it. New and innovative things are the future.