@lillycoyote I don’t deny that the specifics of the assignment could change whether or not it is legitimate to not argue for the thesis that Daisy represents the American dream. What I am arguing is that conditional statements can make perfectly good thesis statements.
If I want to write an essay giving a Freudian analysis of “The Fall of the House of Usher,” I don’t have to begin with a defense of Freudian analysis. It’s just one way to read literature. What I need textual support for are the specific claims that I think that lens brings out (e.g., “Roderick and Madeline are in an incestuous relationship”). But my thesis statement here is something like “a Freudian analysis reads Roderick and Madeline as being in an incestuous relationship.” This could be construed as a conditional thesis as follows: “if we limit ourselves to a particular analysis—viz., a Freudian one—then Roderick and Madeline are to be read as being in an incestuous relationship.”
Similarly, I think “Daisy represents the American dream” is the lens in this case. Just like in the above case, then, it is not strictly necessary to defend the statement “Daisy represents the American dream” in order for “if Daisy represents the American dream, then Fitzgerald is saying __________ about American society in the 20s” to be a legitimate thesis. Maybe @jballzz‘s teacher would not allow it for this particular assignment, but conditional statements are not illegitimate as thesis statements per se.