^^^Call it a chain reaction of events. If Kirk hadn't taken command, Spock might not have been there, and McCoy definitely wouldn't be there (however inconsequential his presence was). Decker might have been distracted by Ilia's presence on the bridge and made the wrong call during the "lightning" scan. That might have caused Enterprise to be digitized, and there would be no one to decipher V'Ger's origin, and no one to enter the transmission code. Earth would have been destroyed.
Well, we don't know that Spock wouldn't have come back had Kirk not been there. He was already in communication with V'Ger before anyone knew Kirk wanted the Enterprise
back. If Decker had stayed in command, it's possible Sonak doesn't die in the transporter accident because Decker doesn't rush the ship's launch, making Spock's presence as science officer unnecessary, but that doesn't mean he couldn't have come along as a special Starfleet observer. Decker certainly knew his background and probably would not have objected to his being present for the mission. What Decker may have done after Spock's spacewalk is another matter, however.
Kirk's presence made the difference in TMP not because of his tactical brilliance (not that he's Picard or Sisko to start with) but because of his guile and determination. How many captains would have had the guts to enter V'Ger despite knowing next to nothing about the entity? Picard and Sisko would likely have done the same thing (perhaps for different reasons), but would someone like JT Esteban have taken the chances that Kirk did? I doubt it.