I believe I read in the Exo Grimoire that the exos were humans who wanted immortality. In this case it doesn't seem like Cayde had much of a choice.
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That was more a theory. An Exo (either Warlock or Cryptarch judging by insight) was giving his/her thesis on why Exos came to be, all the while roasting any imaginary argument the non-Exo student had against the theory ("The stink of anthropocentrism"). At the very end, all outliers are covered and the Exo's theory is presented: Someone wanted to live forever.