I do not even believe fiats will fail. They have not failed over the years and governments have a way of still pushing it out there. However, with respect to your statement of crypto being volatile, this is still a new space and of course the currency aspect of it is not being utilized fully which is what makes it a whole lot speculative in nature and results to the increase in volatility.
For what it is worth, I believe this is something that would change over time, and for sure will become an alternative to fiat, but cannot be for everybody just as you said in some people feeling it is a whole lot complicated for them and would rather stick to fiat.
Fiat will fail, I know that there are many people that will not believe it until they see it, but if you take the time to look at the history you will see that no fiat currency has ever survived and that every single one of them has gone down to its intrinsic value which is zero, I know it may seem difficult to believe because fiat has been around for a long time, but the system cannot last forever and once it crashes a lot of people are going to be burned by it.