It's not that black-and-white, although your point is quite true in a general sense. The denarius underwent a slow debasement toward the end of the Republican period. By the time of Augustus, it fell to 3.9g, or 1/84th of a Roman pound. Nero reduced it to 1/96th of a pound and after that the fineness also diminished consistently. By the 3rd century the weight had dropped to 3g. So the antoninianus can be seen as a natural progression of the denarius' slow "decline" into fiat money, if you insist on viewing fiat money as something negative. But all bronze coins are fiat money, as their bullion value is only a tiny fraction of their face value, and that value has to be guaranteed by the state. In fact, the invention of fiat money by the ancient Sicilians was a huge technological advance. If we all agreed that a medium-sized lump of stamped bronze was worth some small fraction of a drachm, we wouldn't have to risk losing these Lilliputian coins... MYSIA. Kyzikos. c. 450-400 BC. AR Tetartemorion, 0.3g, 7mm, 6h. Obv.: Forepart of boar left, tunny fish (tuna) to right. Rev.: Head of roaring lion left within incuse square. Reference: SNG France 373.
Why not? one can see things like this... But, after all, the antoninianus can be seen as a successful innovation, it was the first coin with which one could make column-like stacks thanks to its large size and low relief. You could not do it before with aurei or denarii, the relief was too high: coins had to be poured in heaps on the counter, we can see it on this Pompeii fresco or this relief from Neumagen. After the antoninianus, all coins were low relief (solidi, for ex., and all medieval, modern and contemporary coinage). .
