What could Caesar gain by staying away? As you say, Cimon, he could have kept his command against any expeditionary force sent to relieve him. But so what?

It was not Caesar's aim to be governor of Gaul forever. He wanted to go back to Rome and be consul again; and not to be appointed consul, but to be elected one, which was, of course, contingent on the will of the people...

The reason he wanted to extend the proconsulship until the election was to keep from being immediately prosecuted in court by Cato, Favonius and their friends. We have no clue how he intended to prevent them from prosecuting after his consulship had ended - probably he meant to have another proconsulship, perhaps in Syria? But the fact was, he wanted to run for consul in absentia and it was the Senate's refusal - or, as Heraklia points out, the consuls' refusal - that brought him into Italy.

Cimon's perspective that we should look for those who profitted from the war - or those who thought they would - is a very valid and interesting one.