Firstly Sparta was destroyed way too easy in this timeline. Granted it wasn't as powerful as in cannon at this point, but still it was strong enough to resist against a greater force.
Secondly there were more Spartans. Way more. In just the earthquake of 464 BC 20,000 people died. Historians estimate that at its peak there were 20,000 – 35,000 free residents and 15,000-20,000 helots and perioikous. So we can assume that about 20,000-30,000 people survived. And keep in mind, the earthquake happened 16 years before the destruction of the city. Meaning that they had plenty of time to grow their population and even train it.
Sorry my friend but i strongly disagree. First of all when i decided to make this Athenian hegemony ATL i searched the dates and the history of the era to find the best date to start this ATL, with Athens really strong and Sparta weak. So the dates after the earthquake of Sparta and before the disastrous Egyptian expedition of Athens was a perfect for stronger Athens ATL.
Sparta population peak was around 500 BC and from then their numbers start to decline(Their laws meant that Sparta could not readily replace citizens lost in battle or otherwise and eventually proved near fatal to the continuance of the state as the number of citizens became greatly outnumbered )
The Spartan OTL crisis, that i took advantage in this ATL, began in 465 B.C. with a tremendous earthquake in Laconia, the territory of the Spartans in the Peloponnese. It killed so many Spartans(maybe even 20000 as you said, although most modern scholars don’t agree with this numbers) that the helots in Messinia and even in Laconia instigated a massive revolt. By 462 B.C. the revolt had become
so serious that the Spartans, swallowing their considerable pride,
appealed even to Athens among for military help, despite the chill that had fallen over relations between Athens and Sparta since the days of their cooperation against the Persians. All this shows major issues in the Spartan manpool. Lastly about the 16 years, its not even a full generation(20 years) to fill up their ranks.
Fourthly, during the first Peloponnesian War there was only one significant battle between Athens and Sparta. The Battle of Tanagra in 457 BC. From what I gather this one didn't happen in this ATL but it is worth mentioning that facing a force of 14,000 Athenians, the Spartans won having only 11,500. Those 14,000 were their whole army according to Thucydides with 1,000 men from Argos as well. Considering the more battles Athenians have engaged in this timeline it is most likely that this number has been significantly reduced, and there are more mercenaries than Athenians. While the Spartans being more isolationist have more. Moreover, in the battle of Tanagra the Athenians controlled the upper ground as we could say. All in all, against higher numbers, against the field advantage the Spartans won.
When this battle happen in OTL the Athenians had also 200 triremes in Cyprus/Egypt (40000 men) where in my timeline this expedition was cancelled… Also in the battle(Tanagra) the Spartans won the battle, but they
lose many men and they were unable to follow up on their victory(We can also note that the Spartan where only 1500 in this battle). All this are again indication of small numbers.
In my timeline Athens had a man pool of more than 60000 citizens to draw troops, plus extra allies in the likes of Corinth, Boeotian league and most of Achaian towns. Now If you combine all that with Thessaly,Argos and the Delian league members…
Sparta on the other side had Elis/Eleans and Arcadia(who they didn’t really like the Spartans, at least a big percentage of them).
Then we have the battles of this timeline:
460 BC
Argos rises against Sparta. Athens supports Argos and sends 2000 troops. The small force that is sent by Sparta(due the helot revolt/siege of Ithome) to quell the uprising in Argos was defeated by a joint Athenian and Argos force at Oenoe.
Small losses for each side
458 BC
The two armies met near Megara. Although the Athenians outnumbered Spartans 2 to 1, both sides suffered heavy losses(more than 4000 for each side) and the battle ended in a stalemate. After the battle, the Spartans, marched home across the isthmus.The Athenians rebounded well after the battle, by sending an army under Myronides to siege Corinth.
Heavy losses for both sides with Sparta way harder to replace them.
458 BC
in another small battle the Athenians attacked the chief Spartan port of Gytheion and burnt the dockyards.
Small losses for each side
449 BC
The Spartan alliance held the field and the Athenian alliance retreat in order on a large near by hill. Both sides suffered heavy losses. More than 7000 Spartans/allies and 9000 Athenians/allies killed in the battle. After a while the Spartans decided to lift the siege and return to Sparta.
Again heavy losses for both sides with Sparta way harder to replace them.
* Athenian alliance: 27000(Argos 5000, Thessaly 5000 ,Thebes/Boeotians 6000 Athenians 11000). Spartan alliance 19000(4000 Spartans, 8000 Arcadians, 7000 Eleans)
Now when Athens invaded Peloponnesos with their massive army, the Spartans unwilling to engage the massive army in battle, simply defended their city. Then the Athenians
liberated some of the Lacedaemonian perioeci from their allegiance to Sparta.
So in my ATL during the siege, Sparta didn’t have more than 30000-35000 people(including women,old people and children)
All in all, it is a political nightmare to destroy such an important city. They were the ones who held the ground against the Persians. And while Athens is certainly strong, that doesn't translate to all powerful, they still need the alliance of other cities and if their allies saw what they did to former allies and even brothers in arms (from the Persian Wars) then they would be skeptic to say the least, in allowing Athens to expand even more.
Now I am not completely certain whether Pericles had the full support of the Athenian government, but they would never in any way commit to such an atrocity. Remember there was a common idea that all Greeks were civilized and the rest were barbarians (even before they were united due to their achievements against stronger forces). Razing such an integral part of the society was against the interests of Athens. If i recall correctly there have barely been 2-3 razed cities from one part of the ancient Greece to another, and it is more along the lines of utter betrayal (for example a city allied to Sparta sells them to Athens but then loses and is being razed).
Yes i agree the destruction of Sparta was an atrocity. But this started first of all from the Spartans themselves who not willing to surrender, fought from door to door and burn most of the town by themselves. Also lets not forget that the radical democrats in Athens, all this years have proclaimed Sparta as their worst enemy, an enemy of democracy. So in the Athenian troops were some really fanatic "democrats". Now combine that with the Messinian troops in the Athenian army who where seeking revenge, and the furious resistance of the Spartan within the city. Visualising this scene i can see Pericles giving the order for the final blow.
Pericles as a political animal wanted also to take the spot of Ephialtes in the radical democratic party. So after the success in Cyprus, he decided to stay in Laconia to eliminate the other major threat to Athenian democracy that would make him the undeniable leader of Athens. Did he planed to destroy the city? No, but sometimes shit happens...