Bret Devereaux

One other possible problem: the pike phalanx evolved during the wars of the successors, particularly longer heavier pikes and smaller shields. It is likely these developments made it better for pike on pike combat.

But the Hellenistic phalanx was probably less agile and flexible than Philip and Alexander's phalanx. And they put the pikemen at a disadvantage if their direct charge dissipated.

One thing that is notable: the pike phalanx, despite its successes against the Persians, is not widely copied outside of the successor dynasties. Only a few Greek states (Acheans, Sparta) try to field pike units, in both cases to fight the Antigonids.

But by the 160s, a lot of folks are trying to imitate the legion, adopting mail and gladii and even trying to mimic the organization with centurions and triplex acies. This includes big powers like the Seleucids, and small ones like the Hasmoneans (Sekunda 2001)

Are we sure that the Argyraspides literally copied Roman equipment? Or did they just use heavy Galatian or Thorakitai equipment since that already existed?

On the one hand, it's hard to imagine how the "picked thorakitai" of Ptolemaic Egypt (attested 197 BC) would have been all that different in appearance from legionary heavy infantry. But the Roman organizational scheme and adoption of gladius probably set apart the Daphne 5000

Yeah, the thing about the Daphne procession is that Polybius is explicit on the 'Romanness' of the picked 5000, which I think argues against reading their equipment as being understood as culturally Galatian.

Personally, I think this has to do with the fact that they were mailed infantry. We have a decent amount of evidence for Gallic mounted elites wearing mail, but unsurprisingly such an expensive armor seems to have not been used by the infantry.