Digital payments make transactions much easier, but despite that, fiat currency will never disappear because digital payments cannot be made by everyone and everywhere, for example in traditional markets or by elderly people, and I agree with what you said, it only decreases, not that it can be completely replaced.
I think you are referring to "cash" (instead of Fiat), and yes, it will disappear sooner or later. Fiat, however, will be with us forever since it's the "tool" governments use to manipulate the economy. Without Fiat, governments will lose the power/control they currently have. A cashless society is not only about convenience (fast, cheap payments), but also giving banks and governments greater control over your financial life. They will be able to freeze/confiscate money or even change monetary supply "on the fly". With existing centralized payment processors (VISA, Mastercard) and cash, this is difficult to do. CBDCs aim to fix that by "digitalizing" money as we know it.
Cash will become a thing of the past once it's no longer "legal tender". This could take from a few years to a few decades. Fortunately, we have Bitcoin. As long as it remains decentralized, we should have nothing to worry about.