We can't be sure; any argument as to the role he would have played, had he not been recalled to Athens, is hypothetical. It is certain that Thucydides portrayed him as the brightest mind in the council of generals that was sent to Sicily, and appeared to endorse the strategy he suggested (in which the Athenians would subject all the smaller states of Sicily first, enlarging their army and resources, before descending on Syracuse). However, it is not certain that he would have been able to persuade Nikias and Lamachos to follow his advice. Nikias had the seniority; he was a far more experienced general than Alkibiades, who had only started playing a role in Athenian affairs a few years ago. Lamachos, too, was a veteran of two decades of campaigns. These generals could outvote any of his proposals in council and wouldn't have liked being bossed around by the upstart. So even if Alkibiades hadn't decided to flee to Sparta, it's not certain that he would have been able to influence the course of the campaign.

The focus of Thucydides on Alkibiades' supposed genius, and of later sources like Plato and Plutarch on his personality and extraordinary abilities, lead us to lose sight of the fact that he wasn't a singular military mastermind in a world of incompetent bunglers. Thucydides almost certainly gives him too much credit when he claims that he advised both the Spartans and the Persians on the strategies that won them the greatest advantage. Rather, we should recognise that Alkibiades was an ambitious and charismatic young commander, but one with an already ambiguous track record (given that he involved Athens in the disastrous war of Argos against Sparta in 420-418 BC) and no major victories to his name. The other generals sent to Syracuse - Nikias, Lamachos, Eurymedon, Demosthenes - were the most experienced and successful Athenian commanders of the time, whose track record included the greatest victories of the Archidamian War. It is too easy to argue that the Sicilian Expedition lost its brain when Alkibiades was recalled. It is more realistic to argue that the campaign proved too great a challenge for even the best tacticians Athens could muster - and Alkibiades may not have made much of a difference if he had remained among them.