They have reasons to believe that it will be gone, given that the current state of research on CBDCs and the overall trends and interests in cryptocurrencies around the world is improving. More and more huge companies, together with banks and governments, are recognizing the potential of digital currencies if it is integrated in the economy, hence why Deloitte thought that maybe the end fiat is nigh. Perhaps their projection is very sudden, but I can see that fiat will soon be outdated and replaced by digital currencies.
As far as i know, even if digital currencies will become mainstream and be adopted legally, i don't think fiat will be totally gone. They will definitely stay and maybe digital currencies will only be integrated into fiat so that fiat will still be functional in the long run.
I know i am very enthusiastic and optimistic about bitcoin and the rest of cryptocurrencies but admit it, fiat is still benefited by the most part of the world so we can never really tell that 5-10 years from now, fiat will lose its value and will be replaced totally by digital currencies.