Even rich people who have fiat but do not have any bitcoin at all, do not keep their money in fiat itself. They just buy investments with it. It could be real estate, it could be gold, ti could be stocks, bonds, whatever you can think of.
You are not going to see any person with 1 billion dollars in cash at some bank, it could be as liquid as it gets in some bonds if needed be, but never in cash, like they are not going to put up 1 billion dollars in savings for a 3-5% return or something. This is why nobody who has any decent amount of money keeps it as fiat, they all put it in some investment. The difference is, if you have 100 dollars, then investing in anything at fiat makes no sense, in crypto? It is great.
They have their own way of making a profit although not always in bitcoin, although I doubt it today, I think the billionaires must have bitcoin only the amount is not large as in other investments, bitcoin ownership can be confidential.
You may not know that there are a lot of money games to get big profits even through banking, such as you buy the money into an asset, then you pledge it to a bank that has a low interest rate to get money like for example a Japanese bank and you deposit the guaranteed money into a bank that has a high interest rate like a bank in the US, you can take the difference to pay and make a profit there.
If I only have $100, I would rather invest in myself, enrich my knowledge or improve my skills to get more money and then talk about investment and bitcoin for today can be the main choice.