I agree that “only evolved once” (and then “subsumed the market”) is not that much evidence for contingency on its own. But if you combine it with the knowledge that its evolution was facilitated/enabled by some random-ish, contingent factors, then the contingency case is much stronger. For example, the great diversification of mammals was enabled by the extinction of dinosaurs, itself caused by an asteroid impact, a factor as contingent as you can ask.
I agree that “only evolved once” (and then “subsumed the market”) is not that much evidence for contingency on its own. But if you combine it with the knowledge that its evolution was facilitated/enabled by some random-ish, contingent factors, then the contingency case is much stronger. For example, the great diversification of mammals was enabled by the extinction of dinosaurs, itself caused by an asteroid impact, a factor as contingent as you can ask.