[subungual]

Yeah, your logic is pretty good here. Prions are a super interesting emergent phenomenon of a complex system. In a weird way, like cancer or viruses, they're just an inevitability of having enough of certain materials interacting for long enough time. Proteins misfold in just such a way that allows them to induce the same type of misfolding in other proteins, leading to propagation. It's less a definitional thing, and more a matter of the fact that something more stable will persist longer, encounter more opportunities to propagate and preserve its form. That said, surviving harsh environments isn't an entirely necessary condition, as not all prion generation requires transmission. Unfortunately, if you have the wrong gene(s) broken, you're at risk of just generating them yourself. That said, transmission is significant, and since it involves needing to be able to survive a harsh outside environment with no propagation between hosts, extreme durability is a definite boon, as you suggest.