I think either could be right as there are no proof for any of them. We should just leave it to Sampaoli and his team of analyst as they must've considered objectively, not subjective.
Also, Argentina must change their play. By saying, a system that can fully support Messi as center of play, it's like.. "here, leo, We'll hold them off, you go with the ball", and leave Messi alone as core of attack, team with aguero for goal. Even, Messi is changing his style of play, to more of a supporter rather than finisher (ended up top finisher anyway) at Barca.
Argentina need a team that have people who work effectively according to each of their own position and collaborate well. Not relying Messi for everything. He could still do it, like in the group stage against Iran, Nigeria last time. But not at knock out, too much burden.