This reasoning is so flawed that I do not even know where to begin to deconstruct it!
I might start by recommending some reading that has been missed by the majority of academics in biblical studies. If we are really going to talk about whether or not constructed myths have any historical value (which seems to me to be the ultimate goal of TJP), then we better get up to speed fast on what other fields are saying about it. See below a selection of publications that I have found particularly helpful in terms of my own work - but this is just the tip of the iceberg. The field is vast and growing with a huge bibliography.
What Social Memory theorists have found is that all societies create their memories to support their present experiences and to help them move forward into the future as they perceive it should be. This is usually done by taking historical figures and events and reframing them into older myths or legends, and by keying them to older personages and ideas. The best example for us (because we can witness this happening now) might be with Barack Obama. He is already keyed to Martin Luther King, Jr. and to John F. Kennedy and Abraham Lincoln. His own historical story is being framed in terms of our collective memory of Lincoln - so we accepted without too much discussion his train-ride into Washington, D.C. and his use of Lincoln's bible at the inauguration. His story of rising from a middle-class family to the presidency is already shaped by the story of Lincoln's rise from the log cabin to the White House. We are understanding and interpreting Barack Obama (and what we hope he will do) within a certain set of stories and myths from our past.
So this is what we do as communities. This is how our minds operate. We understand our history and what is happening in our present by casting it into familiar forms and tropes. This helps us to deal with things and make sense of things. It gives us hope and meaning. It is a natural process of the operation of our minds and human memory.
But because we do this, does not mean that there is no historical value to our stories. All these people lived and did things. Abraham Lincoln, John F. Kennedy, Martin Luther King, Jr. and Barack Obama. In fact, it is most often the historical people that do great things which we attempt to reframe and mythologize! The people that are average Joes (unless you are Joe the Plumber!) fall into obscurity.
Yes our gospels are theological treatises. Yes our gospels are mythological in their framing of Jesus. Yes our gospels present us with different portraits of Jesus, as do modern scholars who work on the historical Jesus. But none of this suggests even remotely to me that this means that Jesus did not exist as a historical person. In fact, when understood within the communal memory-making process itself, the fact that a Jewish crucified criminal is mythologized as a god that the Romans should embrace as God is highly suggestive that there was such a man, and that there were a group of people who understood whatever he did to be extraordinary. And so they framed and keyed his story with those they already knew, from the Jewish scriptures and from the Greco-Roman classics. And a historical person became an angel and then a god (at least that is my operating hypothesis).
Jan Assmann, “Ancient Egyptian Antijudaism: A Case of Distorted Memory,” in Memory Distortion (ed. Schachter; Cambridge: Harvard University, 1995) pp. 365-378.
Anthony Le Donne, “Theological Memory Distortion in the Jesus Tradition,” in Memory and the Bible in Antiquity (eds. Loren T. Stuckenbruck, Stephen C. Barton, and Benjamin G. Wold; WUNT 212; Tübingen: Mohr Siebeck, 2007) 163-177.
Doron Mendels, “Societies of Memory in the Graeco-Roman World,” in Memory in the Bible and Antiquity (eds. Loren T. Stuckenbruck, Stephen C. Barton, and Benjamin G. Wold; WUNT 212; Tübingen: Mohr Siebeck, 2007) pp. 143-162.
Barry Schwartz, Abraham Lincoln and the Forge of National Memory (Chicago: University of Chicago, 2000).
Barry Schwartz, "Collective Memory and Social Change: The Democratization of George Washington," ASR 56 (1991) 221-236.
Barry Schwartz, “Memory as a Cultural System: Abraham Lincoln in World War II,” American Sociological Review 61 (1996) pp. 922-923 (908-927).
Barry Schwartz, “The Social Context of Commemoration: A Study in Collective Memory,” Social Forces 61 (1982) p. 393 (374-402).
Barbie Zelizer, "Reading the Past Against the Grain: The Shape of Memory Studies," Critical Studies in Mass Communication 12 (1995) 214-239.
Yael Zerubavel, Recovered Roots: Collective Memory and the Making of Israeli National Tradition (Chicago: University of Chicago, 1995).
Yael Zerubavel, “The Death of Memory and the Memory of Death: Masada and the Holocaust as Historical Metaphors,” Representations 45 (1994) 72-100.
Yael Zerubavel, “Antiquity and the Renewal Paradigm: Strategies of Representation and Mnemonic Practices in Israeli Culture,” in On Memory: An Interdisciplinary Approach (edited by Doron Mendels; Bern: Peter Lang, 2007) pp. 331-48.
Yael Zerubavel, “The Historical, the Legendary and the Incredible: Invented Tradition and Collective Memory in Israel,” in Commemorations: The Politics of National Identity (ed. John R. Gillis: Princeton: Princeton University, 1994) 105-125.