I don't think its fair to say that Decker in command the mission would have failed. I mean consider what happens if Decker is not there. The Enterprise is destroyed by a asteroid we Kirk orders a phaser strike and nothing happens. Decker could have succeeded, it would have likely done things way different. Now hay Kirk is the man, but he leads without much discussion at times, its based years of command. Decker seems to be more a democratic leader. It says a lot about him that when he gets canned by Kirk he is down in engineer working with Scotty. I don't think Kirk ever did something like that. Its not to say one is better than the other. Decker would have launched the Enterprise slower but in perfect working order. He would likely consulted his command crew before proceeding, and would likely found a way to integrate Spock into the crew. Might have keep a closer watch on him than Kirk did.
Also, all Decker's opinions and suggestions may not really be the actions he himself would have taken had he been in command. Possibly. As he says, "As your exec, it's my duty to point out alternatives." As the first officer, he has the option of presenting actions to the commanding officer. If Decker had the center seat and ultimate responsibility, maybe he would have done things differently. There could have been a difference between his suggestions fulfilling his role as exec versus his actions as captain of the ship. Just saying. Maybe.
Also, Kirk towards the end seemed pretty confident that suicide bombing with the ship's antimatter engines would be a workable solution to the V'Ger problem. Now, that's a Decker family tradition right there! OTOH, Kirk is famous for only using suicide as a bluff, and shying away if actually confronted with it ("By Any Other Name"); Decker might be more willing to take that step. Especially if the loss of Ilia comes to pass in this alternate story as well, and leaves Decker in grief. Timo Saloniemi
I think Decker would have been fine. And actually, when you think about it, Decker was absolutely right about two things: Kirk had practically no experience at all with the new Enterprise, and he had not logged any star hours in 2.5 years (could have been even longer since his last command). What would have made sense is Admiral Kirk being aboard as an adviser with some measure of veto to Decker's decisions, but NOT as Captain. But then of course, it wouldn't have been quite like the "good ol' times". Kirk would have been along just for the ride. However, he'd have gotten his captaincy at the end anyway, with Decker vanishing.
I suppose Admiral Kirk could have commanded the mission and Captain Decker the ship. There might have been even more dramatic possibilities there. At the end of the film, after Decker is lost, Kirk could have asked to be demoted to replace Decker. Could have worked.
I'm sure Decker would gladly have accepted his help. Spock was a legend by then. When Spock comes on the bridge, Decker looks to be in awe and gladly steps aside from the science station.
Decker still would have taken Spock up on his offer as science officer, Spock never offered to be Chief Science Officer, just science officer. Whoever Decker already had as first officer would have stayed (Sulu?). If McCoy hadn't been along for the ride would it have made any real difference? He didn't contribute anything that Decker himself would have found useful.
Do we know there is even a position called "Chief Science Officer"? Throughout TOS, Spock was always referred to simply as "the science officer." And when Sonak is killed in the transporter accident, Kirk doesn't pick a junior science officer to fill the role. He gives it to Decker. And says "you'll have to double as science officer." The implication seems to be that there is only one. Of course, we know that there are other scientists aboard in the different departments. But it seems there is only one person designated for the post of "science officer."
The thing about the term "science officer" is that it's a fairly generic term. There are multiple sciences--life sciences, planetary sciences, stellar sciences, social sciences, etc.--and each could have its own department headed by junior officers that report to a single officer in charge of them all. In that sense, the term "science officer" could be simply shorthand for "chief science officer" (or as I personally prefer, "chief sciences officer").
^^^ Which makes me think of something... If each science has an officer in charge of that discipline, then a "Chief Science Officer" may not even need be a scientist. CSO could be an administrative head or department executive, with the chief science responsibilities headed by Chief Geologist, Chief Astronomer, etc.
I think this is more because only Sonak and Decker were trained in operating this new type of starship. A generic science officer would not know his, her or its way around the controls and interfaces. (Of course, Spock has no problem here, being Spock; those halfbreeds have to be prepared to outdo regular folks at every turn!) It could be argued that in "Where No Man Has Gone Before" already, this was not the case. In the staff meeting where dealing with Gary Mitchell is discussed, Lieutenant Dehner first indicates she has failed to objectively evaluate Mitchell, then accuses Spock of being heartless when he corrects her on the issue. Kirk then chides somebody by saying "It is my duty, whether pleasant or unpleasant, to listen to the reports, observations, even speculations, on any subject that might affect the safety of this vessel, and it's my science officer's duty to see I'm provided with that." Since it's Lt Dehner who has failed to provide Kirk with this intel, we could well argue Kirk is chiding her for having failed in her science officer duty. After all, while Spock was also involved in the "seeing to providing", sort of, he doesn't yet wear Science colors in that episode, whereas Dehner certainly does. Which would be welcome news in explaining how Spock can so easily assume the role despite "originally" wearing command gold. Although of course he did have a background in sciences from the first pilot episode. Timo Saloniemi