It would be incorrect to think that the sarissa phalanx was regarded as outdated at the start of the Third Century BCE by either Pyrrhus or the other non-Macedonian powers. Our sources on Pyrrhus suggest that he did make use of the traditional phalanx even after his first encounters with the Romans. Polybius, for example, writes (Histories 18.28): "Pyrrhus, moreover, made use not just of Italian armaments, but even of Italian formations, aligning a maniple alongside each division of phalanx-soldiers within his formations in his battles with the Romans." Beyond this, our evidence is mixed. We know, also from Polybius, that both the Achaean Confederation and the Sparta of Cleomenes III adopted the sarissa phalanx in the final decades of the Third Century BCE. On the other hand, we do have good evidence for the adoption of divisions that seem to be at least comparable to the maniple, as a mobile mass of heavy infantry, in the Ptolemaic and Seleucid armies of the Second Century BCE.

Why, specifically, did Philip V retain the traditional phalanx? The question as it is posed demands a small measure of speculation, but grounding this we might say: (1) Philip V, at the start of his reign, could look back on decades of internal Greek warfare in which the phalanx had proven itself. A Spartan sarissa phalanx fought a Macedonian sarissa phalanx at Sellasia in 222 BCE. Macedonian phalanx-soldiers had stood at the front, it would seem, throughout the wars of Antigonus Gonatas, Demetrius II, and Antigonus Doson; (2) his own early experiences in the Social War of 220 - 217 BCE did not undermine the status of the phalanx; (3) when in need, the Antigonid-Macedonian armies did have alternatives that were more comparable to the Roman maniple, like the peltasts, descendants of the Argyraspides of Philip II and Alexander III, themselves armed similarly to hoplites, or the other light troops drawn from the Balkan frontiers of Macedon; (3) it is not clear that the wars of Philip V would have suggested to a prudent observer that the phalanx was inferior to the maniple in the manner suggested by Polybius (that is, stressing the ability of the maniple to maneuver over rough terrain and for its junior officers to make spontaneous decisions). There were, in fact, numerous moments in the First and Second Macedonian Wars where phalanx-soldiers proved their worth. Consider the following episode from the Second Macedonian War as recounted by Livy (the translation is that of Yardley): "The Macedonians, in close order, held out before themselves their lances of extraordinary length, while the Romans hurled their spears to no effect against the solid mass of their shields, built up into what was virtually a tortoise formation (32.17)." Granted, in this passage the Macedonians defend a breach in a wall such that their flanks are not exposed. Still, when would the phalanx have proven itself inadequate in the time of Philip V? The Battle of Cynocephalae of 197 BCE was a meeting engagement, making it harder to suppose the true competencies of either force than it would be later at Pydna under Perseus I.

In summary, my own knowledge of the texts would lead me to draw into question your premise, that the Macedonian phalanx would seem to be outdated to a Greek observer in around 200 BCE. That conclusion awaited a set piece battle like Pydna.

Bibliography

Gruen, Erich. The Hellenistic World and the Coming of Rome (California, 1984).

Will, Edouard. Histoire politique du monde hellenistique 323-30 av. J. -C. (1966).