You say: “there seems to be an underlying assumption that all participants in the thread seem to be making. The assumption being that all these texts have the same end goal in mind, and that this end goal is enlightenment/eudaimonia/happiness. I submit that such an assumption is unwarranted.”
It is not an assumption but an explicit hypothesis, as far as I am concerned, that all the Indian traditions (not just these two sets of texts, namely, Upanishads and Puranas) are concerned with the same end goal. I want to provide a description of the Indian traditions as attempts to help human beings find happiness on earth. (For the time being, in order to avoid fruitless controversies and to have a point of departure, I take Aristotelian Eudaimonia to be an explication of this notion of happiness.)
Assumptions often do require warrant, but hypotheses do not. Because the truth of a conclusion depends on the truth of the premise (often, assumptions function as premises in an argument) provided valid rules of inferences are used, it makes sense to ask for the warrant of an assumption. Needless to say, this condition does not hold for hypotheses: you have to evaluate them entirely differently.
Of course, in one sense, such a reconstruction would be the hypothesis I propose. Until such stage, the belief about the commonality of the goal of the Indian traditions would be an assumption. However, it is not a premise in a logical deduction but one which functions as a heuristic. Consequently, if I need to convince you to accept this assumption for the time being, I need to show you what is cognitively interesting about this assumption.
It promises to help explain the rivalry within the Indian traditions; it sheds new light on the inter-relationship between them in a different way than as ‘syncretism’; it helps one look for a set of problems underlying the Indian traditions; it allows one to formulate these problems in the twenty-first century language; it helps one evaluate the answers of these traditions and even carry them forward. These are some of the cognitive benefits of this assumption.
Of course, ultimately, it might turn out to be the case that no such goal is to be found and that the Indian traditions are not each other’s rivals. I do not know. For the time being, this idea appears very productive. That is enough of a warrant. .