Nothing Jason achieves is achieved by Jason. The fleece is won by Medea, the escape is managed by Medea, and the usurping uncle at home is disposed of by Medea, who persuades his daughters that they can rejuvenate their father by cutting him up and boiling him. Ancient audiences noticed. He is a hero whose defining characteristic is that he is helped, and the tradition is unusually unkind to him about it.
The ending is the reason the story survives. Established at Corinth, he sets Medea aside to marry a local princess, on the argument that the match is good for their children. In Euripides' version she kills the princess, kills her own two sons to leave him with nothing, and departs in a chariot sent by her grandfather the Sun, above him, safe, while he is left in the street. Whether Euripides invented the murder of the children or found it in local tradition is argued; either way it is the version that took.
Jason dies obscurely, killed by a beam falling from the rotting hull of the Argo while sitting under it. The Greeks liked that ending for him. A man who won everything through an oath he then broke is finished off by the wreck of the thing that carried him, and the poets did not have to add a word of judgement.