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.