stupid generals within history

Yeah there's some truth to that. In fairness though, Alcibiades didn't defect to let Nicias face the music, he was arrested for mutilation of the Hermai (a crime of which most historians consider him likely innocent) which would have left Nicias in command anyway (there was a third general Lamachus, but he was generally overruled by the more influential Nicias and Alcibiades). So he had a choice between being sentenced in Athens or escaping and defecting to Sparta. Once he had defected he aided the Syracusans against the Athenians. Politically he was a snake, but as a military commander he generally did pretty well.

Sparta's success in the final stages of the war was probably more due to their alliance with Persia than Alcibiades.

As for whether the Sicilian Expedition was a good idea in the first place, I suppose hindsight is 20-20. If it had worked, it probably would have greatly expanded the Athenian Empire beyond the modest Delian League since it would have allowed Athens a secure base to take on Carthage. Nicias was the last person who should have been put in charge of it though - he was against it from the start and was an overly cautious commander, unsuitable for a campaign where he would be outnumbered. Had Alcibiades or Lamachus been the sole commander, it probably could have gone better.