Inherently, you are correct about “the dead don’t breed” and thus it would eventually be conveyed generations down the road to absolutely never touch those things. And in either circumstance, being BRIGHT AND OBVIOUS to potential predators right now is a huge disadvantage. That disadvantage is massive if the trait banks on “the next generation is less likely to eat something like this”. Genetic memory isn’t an ideal value compared to the cost, while showing relevance to a non-deadly warning would be adopted faster. And it benefits the non-deadly variety since both warning routes receive selection pressure. Every event is potentially a usage of defense material (venom), injurious, or fatal, so even snakes with deadly venom are benefitted through avoidance rather than needing to expend the venom.