This morden age one don't need holding a fist currency, we can save them as emergency funds and also invest in crypto currency if we want to become wealthy and successful in the future. Fiat currency lose value everyday and holding it will neither do one any good. Imo shitcoins and fiat are like one. Suggesting to invest in Bitcoin for the long term so can make a good profit within 2 circles of Bitcoin.
Both are necessary and we just need to balance fiat is needed for daily transactions and crypto investment must be as a hedge against depreciation, after all if you withdraw crypto you will eventually liquidate fiat for daily shopping.
If there are extra funds and idle is the right suggestion if the word is put into several investment instruments and investing in crypto is one of them to keep the depreciation balanced and continue to grow.