fiat money doesn't have to die for bitcoin to grow and bitcoin growing doesn't have to kill fiat to replace it. bitcoin can and will coexist alongside fiat as time goes by and increases its adoption. all the country debts, shenanigans that we see in the fiat world aren't really going to kill it in my opinion. not to mention that decentralized currency is not some perfect solution to save every problem of the monetary world. it has problems of its own.
I suppose when the crypto are legalize the use of fiat currency will be low. As we see in the initial era people use barter exchange but after the fiat currency was introduced slowly barter exchange was disappear. People now a days need more transparent easy and instant thing so I personally believe that crypto is far better than fiat currency in term of technology and transparency.