We say diversification is key. Depending on your age, financial situation, and overall risk tolerance, you'd allocate different percentages of your portfolio to stocks, fiat, gold, and crypto.
Holding all in fiat is missing out on great opportunities and putting all your eggs in one basket. Holding all in crypto is at this stage generally ill-advised and you'd still be putting all your eggs in one basket.
The next world financial crash/crisis is coming sooner or later, and when it happens, there's not telling what will happen to crypto. Will it grow by 100000%? Will it all go down to zero? No one knows.
Here's something that argues this point a bit better: https://steemit.com/bitcoin/@ledger256/do-you-know-when-the-next-market-correction-is-of-course-you-don-t