The end of the war almost certainly would have been different. Athens continued to fight even through coups as long as it had a fleet. Once Lysander captured the Athenian fleet at Aegospotami Athens almost immediately surrendered. Athens had lost fleets before, but being out of resources in 405 meant losing that particular fleet ended their capacity to carry on the war.

Of course that doesn't mean if Athens hadn't attacked Syracuse it wouldn't have found something else to squander its funds on.

If I recall, Thucydides describes the reason for the expedition as the desire to get more resources to prosecute the war against Sparta. Magna Graecia (or Megale Hellas depending on your language of choice) might have been more populous than the Greek mainland - Gelon, the tyrant of Syracuse, thought he could demand the high command of the allied Greek forces in 480 (he didn't get it) and fought a successful war against Carthage. That's a lot of potential man power.

Politically, the "proximate cause" (as Thucydides would say) was the city of Egesta lobbying Athens for help in a conflict with Selinus. Egesta was an Athenian ally and offered to finance an expedition at the cost of sixty talents of silver (Thucydides says that's enough to fund 60 ships for a month - it's a pretty sizable chunk of change). Of course Athens ended up sending almost twice that....

Thucydides doesn't go into the details of the Sicilian Expedition in the same way that he explores the causes of the Peloponnesian War. Its placement within his work directly after the Melian Dialogue almost imply that the expedition was an act of Hubris and/or the moral decline of the city through years of war. There is also the stories about how the envoys were "duped" into helping. This paints (broadly) a picture, of Athens displaying hubris and attempting to expand its wealth (resources) at the expense of other greeks. It's not an explanation that I have found very satisfying.

You can compare Syracuse to Corcyra. Both were Doric, both were "island" nations, both were outside of the formal alliance system in Hellas, and most importantly, both had fleets large enough that when combined with Corinth and the rest of the Peloponnesian League to change the balance of sea power in the West. Corcyra had 110 ships at Sybota. Syracuse had about 100 ships at the second battle in the harbor (numbers from Wiki).

While I have not seen any expert in the period make this argument. I don't think its to far a stretch to apply the same realist / balance of power theories that got Athens involved in Corcyra to the Syracuse and Sicily. For me the other issue that this could solve would be why Athens put so much on the line in the first place. I doubt very few people felt the Peace of Nicias was going to last, as neither side had been defeated. And the previous peace agreement with Sparta had broken down. A Syracusian Alliance with Sparta could have threatened Athenians dominance at sea, cut the supply lines, and force Athens to capitulate (as did happen with Spartan and Persian Alliance). This let's you frame the expedition not as an act of Hubris, but rather as a fight to maintain control of strategically vital supply routes in the likely event of a renewal and a widening of the the war with Sparta.