Messi didn't play but there is guarantee Argentina would have won if he played either. Plus, they got a red card in the first half you know. It was still going to be a tough game for them.
Apart from that, they already secured their lead in the group in the end. They didn't need the win. Maybe they beat Venezuela anyway but Ecuador are much more powerful. They benefited from it well.
Even Scaloni said it before the match that he wasn't going to feature.
They are managing him just in case he makes a decision to play in the next world cup.