The Sparta of Plutarch bears very little resemblance to the Sparta of Herodotus. By the first century AD, Sparta had become a brutal backwater living according to rigid rituals that the inhabitants exploited to attract tourism. These rituals, allegedly based on the Constitution of Lycurgus, were not Lycurgan at all, but rather the product of much latter law-making.
We know for example, that after the defeat at Leuktra in 371 Sparta fell into a serious decline, and 150 years later King Cleomenes III (235-222) introduced "reforms" aimed at "restoring" the traditional Spartan institutions which had been all but forgotten. This "restoration," however, included many new "traditions" introduced by the stoic philosopher Sphaerus of Borysthenes, who was charged with redesigning both the agoge and the syssitia.
After Sparta's defeat by the Achaean League in 188, Sparta was forced to abrogate it's constitution altogether and live according to the laws of the other Achaean States. An attempt to return to the old ways in 143 BC was, after such a long hiatus, hardly a genuine "restoration" as it would have been impossible to exactly replicate even the already corrupted and altered laws and traditions of Cleomenes III.
But the real decay in Sparta's culture and legal tradition began even before Leuktra. It began not long after Thermopylae with a documented dramatic decline in the Spartiate population.
The Great Earthquake of 464, on the other hand, is an event which allegedly took 20,000 lives in Sparta alone, and its role in Sparta’s decline needs to be re-examined. The accounts of the earthquake are nothing if not dramatic. Pliny claims only five houses were left standing, and there are less credible tales of youths surviving because they ran out of a gymnasium to chase a hare, while the army was saved by being marched out in time. While the details may be hard to credit, I think it is safe to say the earthquake was catastrophic without, notably, impacting the strength of the army.
(2) Stephen Hodkinson, Property and Wealth in Classical Sparta, The Classical Press of Wales, London, 2000.
(3) Thomas Figueira, “Population Patterns in Late Archaic and Classical Sparta,” Transactions of the American Philological Association 116 (1986), pp.165-213.