So in order to protect us from getting whiped out by rogue bacteria from a research lab we MUST ban disaster movies NOW!
Its entirely possible that there were other self-replicating molecules on Earth that evolved first, but DNA simply out-competed them to extinction.
If we ever produce a competitor to DNA that was more 'virulent' than DNA, it would similarly out-compete DNA.
It would be the most pandemic of plagues that would affect every form of life. Even if we could protect our own DNA against attack somehow, it would attack all the bacteria that is symbiotic with us and which we need to survive.
Because its a trope. Can you explain how it gets wildly out of hand beyond speculation?
Unknown unknowns. You can know your specific problem domain inside and out, and still have other things show up that rapidly invalidate your assumptions about the set of possible outcomes.
This is how the plot of every horror/disaster movie starts.