The general assembly of the (male and long-term residents with local parents only) Athenians seems to be so over-confident and aggressive by this point that the odds are against anything short of a major disaster stopping them from making some major miscalculation in the contest with Sparta, even if it's not over Sicily. The amount of being swayed by bravado or their emotions into sudden jumps from one decision to a diametrically opposite one is a bad sign for any hope of a sane, calculating long-term policy - as shown by their shifts of decisions in 428 over what to do to the conquered rebel city of Mytilene on Lesbos which had just been recaptured after trying to get out of the Athenian empire by rebellion. One day they decided to kill all the male inhabitants, enslave the women and children, and burn the city down as an example to others, and voted for this and sent off an envoy to tell their troops there to do this. Next day they decided it was not such a good idea and voted in a hurry to cancel this and sent a second envoy after the first with a counter-order; luckily he got there in time. (It sounds a bit like the sudden, emotion-based, and non-clear-evidence-led abrupt shifts in govt policy in the UK in recent years...) .
There was also the long-term danger once the sensible and calculating (but still imperialistic and anti-Spartan) Pericles died in 430, of the Assembly listening to what it wanted to hear from opportunistic and/or over-optimistic politicians who wanted to curry favour with the majority of voters and seem to represent their gut instincts, not to rely on evidence or what would be likelier to work. Cf the US electing Bush Jr and Trump as more like the 'average Joes' in their opinions than 'smart alec' elitist Democrat candidates as President? This habit of listening to a 'down to earth' and 'ordinary' but hyper-aggressive populist leader was the main criticism of the leading 420s figure, Cleon 'the Tanner' (see plays by Aristophanes) - though he was not that incompetent and it was partly snobbery.
The Sicilian expedition was a step too far - even if it had blockaded Syracuse into surrender, turned its anti-Athenian leaders out, and forced it into the Athenian 'alliance' aka empire it would have caused antagonism at the interference across the rest of Sicily and probably a wide-scale revolt as soon as Athens 'stumbled' or lost part of its fleet elsewhere. This same reluctant submission but brooding hatred and willingness to revolt once it was safe (ie once Sparta could come to help them, with Persian money and ships after 412) occurred in the Aegean islands where Mytilene in 428 had met the sort of crushing defeat that Athens tried to mete out to Syracuse in 415-13. A more aggressive Sparta willing to take more risks, a major defected Athenian general with military flair arriving in Sparta to help them (Alcibiades, recalled and driven into exile by Athens as the Sicilian expedition which he was meant to co-lead got underway, after allegations of the 'blasphemous' mutilation of the Hermae statues by him and his partygoers as a prank), and Persian money all encouraged Sparta to be more active after 413. All these factors, or a combination of one or two of them, could hit Athens even if it took Syracuse - and without Alcibiades it only had second-rate generals once the ageing Nicias died.
But the actual defeat in Sicily was not that inevitable, though starving out Syracuse was less likely by the week as the locals revived in confidence and received a capable Spartan officer (Gylippus) plus local food and volunteers to help them - due to a weakly administered sea blockade and the sheer size of the land approaches to Syracuse making building a wall to fence the city in and cut supplies off impractical. (Another case of big promises about a fail-safe wall proving to be unrealistic...?) Once the reinforcements had helped Syracuse to retake the hills on the approach route to the city, and drive the Athenians off them, and the final Athenian reinforcements had proven inadequate to hold the hills, the Athenians - as advised by Nicias - refused to follow the advice of the reinforcements' leader Demosthenes (Not the famous orator) to give up and leave while they still had control of the harbour and could leave by ship. Defeat in the harbour followed, making evacuation by sea difficult . They had a chance to leave by land, but when an eclipse of the moon (dateable to 27 August 413) gave them a chance to do this while it was too dark for the enemy spies to see them packing up their camp and leaving Nicias followed the advice of the priests that it was a 'bad omen' and refused to move - possibly his worst mistake? And even when the Spartans and Syracusans erected a boom over the harbour mouth, defeated the trapped Athenian navy in the harbour, and cut off the Athenians' sea route to safety, the army (c 40,000 men)could have left quickly by land and headed across country to reach allied cities safely - or mostly safely - had it set out quicker, while the enemy were celebrating their victory, and not waited for a final day. The main part of the disaster, in the sense of the huge losses of men and ships as opposed to just a humiliating retreat and then probably a revolt by Athens' allies paid for by Persia in c.412, was due to incompetence as much as to bad luck - and Athens could still fight on with a rebuilt fleet and stand a chance of winning the final part of the wars in the Aegean until another piece of incompetence lost their remaining fleet at Aegospotamai (near to Gallipoli) in 405. By this point Alcibiades had been allowed back , won more battles - and been disgraced again, unnecessarily. Again, at Aegospotamai the navy was destroyed by a dozy leadership - who did not bother to set scouts to watch for Spartan ships in the Gallipoli strait though their own ships were lined up on an open beach like sitting ducks and the Spartan admiral Lysander was not far away. Alcibiades reportedly arrived to warn the admirals that L was likely to attack, but they hated him as a banished rival and took no notice; disaster followed.
