By 2030, the demand for alternative currencies will rise, with digital currencies eventually replacing cash, according to recent research from Deutsche Bank.

What do you think as a Crypto Expert? What are the challenges? What things should be adopted to replace fiat?

I guess it will be at least after 2030. So it must be. Because we can all see how fast robotization and technology have developed in recent years and this will accelerate in time. And it will be impossible for current paper money to keep up with this technology. We need a faster and more technological payment system as a crypto.
Indeed, if they do not oppose the crypto, it will be enough for adoption. There is no doubt, if they start to use it slowly in society, they will see how good it is than the old system.