I don't think fiat will ever crash. At least not in our lifetime. There will always be a use for it, just like there will (now) always be a use for BTC/alts. We just don't know what those uses will be in the future, yet.
Agree with you it is not possible right now but it is sure that price of bitcoin is going to increase with time so it is getting stronger than cash not only in price but in demand as well, now people use bitcoin more than cash currency because it is easy to use and you will not have to go to bank to submit and cash out your money now with bitcoin you will carry your account in your phone and it will remain secure and safe so it will not crash but the use to become less than bitcoin gradually.