Bret Devereaux

A general can't identify every place where a phalanx has become vulnerable and order a counter-attack. But the Romans have a lot of officers who can.

And as @BretDevereaux points out in his dissertation, the legionary is actually very heavily armored and armed: chain mail, a cast bronze helmet, a relatively long short sword. The scutum is a quite heavy and robust cousin of the thureos. https://core.ac.uk/download/pdf/210598702.pdf …

Thureophoroi in the Hellenistic world are basically light-medium troops; the legionary is undoubtedly a heavy infantryman. At that not doubt gives them a serious edge plunging into a gap in the phalanx.

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)