Jason is a classic antagonist. He does Medea wrong and she sets out to bring him down. He's definitely weak as antagonists go. It's doubtful that an audience would ever think he's going to get the better of Medea.
Even in their first debate about whether or not his second marriage was justified, Jason's rationalizations come off as pretty flimsy. Medea's verbal retorts make mince meat of him. Later on, the crafty Medea skillfully manipulates him helping to take her cursed gifts to his new wife, without even breaking a sweat. You'd think Jason might know his wife a little better than to trust her so blindly. But, nope…the bumbling Jason plays right into her hands.
The conclusion of the play might be one of the rare times that an audience sympathizes more for the antagonist than the protagonist. Sure it wasn't very cool for him to take a new wife. Sure he's insensitive about the whole thing. But Medea takes it too far by killing their children. Even if the audience doesn't feel bad for the disloyal Jason, chances are they might just see more sense in his final argument than Medea's.