Archaic Sparta saw not only the establishment of this new, revolutionary form of government (arguably the first democracy in history), but also a significant flourishing of the arts and trade. Sparta’s most significant monuments (e.g. the Menelaion, the Temple of the Bronzehouse Athena, Kastor's Tomb, the Amyklaion, the royal palaces, the Canopy, just to name a few) were constructed in the archaic period. Sparta’s most famous poets – Tyrtaios, Alkman – lived and worked in the archaic age. Sparta also produced significant works of art and export quality products. (See my entry: Spartan Artists: Not a Contradiction in Terms).
Sparta in the classical period in contrast is characterized by artistic stagnation and such a dramatic end to Sparta’s competitiveness in trade and manufacturing that those who study only classical and Hellenistic Sparta are completely unaware of Sparta’s impressive earlier accomplishments. Indeed, based on descriptions of the Spartan state and constitution written at the end of the fifth century and later, Sparta appears to have become a city-state that disdained luxury and by inference art itself. In short, Spartan society underwent a radical, indeed revolutionary, change in the mid-fifth century BC, immediately following Leonidas' death.
The fact that Leonidas was the last of the archaic kings not just in terms of timing, but in terms of policy is demonstrated by the fact that he took an active interest in world affairs and was elected to lead an international coalition of forces. (See: Leonidas the Diplomat) This fits in well with Sparta's archaic reputation for opposing tyrants and the creation of the Peloponnesian League in the second half of the sixth century. In short, Leonidas' interest in world affairs and his willingness to compromise rather than rely upon brute force were a continuation of the policies of his father and half brother.
Even more significant is the possibility that Leonidas’ domestic policies were tolerant and liberal. We know that in 479, only a year after his death and before his successors could make any significant changes to Spartan policy, the Spartans were able to deploy thirty-five thousand helot auxiliaries outside of Lacedaemon. This suggests widespread support for the Spartan state among the helot population. (The hypothesis that the Spartans took thirty-five thousand rebellious, hate-filled, and untrustworthy armed helots with them, when marching out to face the undefeated Persian army, is ludicrous. Such armed and rebellious helots would have posed an even greater threat to the Spartiates than the Persians themselves!) In addition, in 480 BC Sparta had a fleet of twenty triremes, requiring almost three thousand five hundred helot oarsmen. That is almost forty thousand helots so loyal to Sparta that the Spartiate elite could -- literally -- entrust their very existence to them. Leonidas, if not his predecessors, must have done something to win that loyalty.
Yet little over a decade later, the only recorded helot revolt against Sparta erupted. This is highly significant because we know that revolutions do not occur when people are most oppressed, but rather when rising living standards decline sharply and rising political expectations are abruptly disappointed.
This clearly suggests that in the late archaic period, Lacedaemon's helots enjoyed a slow but steady increase in living standards and political rights. In the post-Leonidas era, however, helot hopes and expectations were bitterly disappointed, leading to the explosive situation that culminated in the helot revolt of 465 BC.
Taken together, the artistic flourishing, international and diplomatic successes and helot loyalty, paint a picture of Leonidas' reign that tells us more about King Leonidas than his death at Thermopylae. While it may be impossible to date the artistic achievements to Leonidas precisely, certainly his role in the anti-Persian coalition and the demonstrable loyalty of the helot population in 479 are a tribute to Leonidas personally and specifically.
Could Sparta’s archaic golden age have continued if Leonidas and his closest companions had not died at Thermopylae? Probably not indefinitely. Athens was on the rise; conflict was almost inevitable. Yet there is little doubt that the helot revolt of 465 BC traumatized Spartan society and set it on a course toward brutal internal repression. The revolt caused Sparta to create despicable institutions such as the kryptea and to commit acts of brutality such as the “disappearance” of thousands of helots who had been led to expect reward. In addition, the helot revolt led to mistrust of Athens and increasingly rabid xenophobia. At least some of the might have been avoided had Leonidas remained at Sparta's helm.
It is not too far-fetched to hypothesize that had Leonidas survived longer, he would have continued the enlightened policies of the archaic kings, who had lived in harmony with the helot population for well over a century. Certainly if Leonidas had lived longer, neither Pausanias nor Leotychidas would have been given a chance to turn Sparta’s allies into enemies.