The claim in the Wikipedia article is not only untrue: it also cites its source falsely. The alleged source is Peter Green's excellent book Alexander the Great, but Green's only comment in the book about "decadence" is to say (p. 125)

'Decadence' is not a popular term these days when discussing Hellenistic history

At another point he approvingly cites a book that "makes a spirited counterattack against well-established notions of Seleucid decadence" (p. 135). The word "degeneration" does not appear in the book. The only exception he's willing to make is that if you absolutely have to use the word decadence, the Seleucid and Ptolemaic royal families after 150 BCE are where you would use it (but a lot of people would end the Hellenistic period in 146 anyway; Green chooses to extend it to 31 BCE).

If you really have to express a preference between periods (and that's rarely a good idea), then culturally, militarily, artistically, arguably in every way except perhaps for one literary form (drama, and even that's only because not much Hellenistic drama has survived) and one philosopher (Plato), nearly everything about the Hellenistic period was more ambitious, more precise, more carefully worked out, more well-informed, more celebrated, more successful, and had better infrastructure than the Classical period. That doesn't exactly make it better: it's more that Greek civilisation was a much larger-scale thing in the Hellenistic period. "Decadence" is just a dreadful misrepresentation used by people who didn't study the period when they were students but still want to look down their noses.

You won't find many modern classicists who would describe it as degeneration. That's a rather moralizing term, one that ties directly to the negative idea of decadence and a nostaligia, one might say, for the Classic period of Greece. The classical ideas were embraced my many later figures, most signitifacntly Augustus when establishing the foundations of Imperial Rome. Classic Greece was the days of Aristotle and Pericles and Aristophanes, or the Parthenon and peity. While Hellenistic Greece was home to Archimedes and the greatest expanse of Greek influence, it was also extravagant, with its foreign gods and power hungry generals. The difference is easily seen in its art. Compara the ordered processions depicted on the Parthenon friezes with the wild, action-packed images on the Hellenstic Pergamum Altar. The critique of "degeneration" is not a historical one, but a moral one targetting the indulgence of empire and departure from the austere roots of Greek culture.

There's a lot more to this, but the short answer is that the Ancient Greek civilisation wasn't the pinnacle it was due to its military. It was due to its cultural achievements: its democracies, its architecture, its dramas, its art.

The 5th century BC was the high point of these, and was the time period that the Greeks ''defined'' themselves, and is also the time the majority of their greatest - and most extant - achievements were made.