Fiat will always be here. I'm speaking of the US, it's just hard to get hundreds of millions of people to accept that. Specially when it's all they've known.
I'm sure that the US dollar will not always be. It doesn't matter that now the whole world accepts this currency. Monetary relations are present in a humans life for not too long. It should also be borne in mind that they are constantly subject to various changes and adjustments. Since the world has already become acquainted with electronic currencies and has partially started accepting them, I'm sure that now it is only a matter of time. Sooner or later, humanity will become so developed that there will be no need for money-paper, including in US dollars.
Of course, at this stage of development, it is still too early to talk about this (above, in a post, I expressed my point of view on this matter), but for the distant future this is a real scenario.