It has already happened once. But you're right, it's unlikely to happen any longer for the reasons you mentioned.
Complex smart contracts are a big risk from a software engineering perspective. I have doubts whether the tools that exist for developing smart contracts are sufficiently mature or the entire way the work is done is sufficiently developed for DeFi to replace traditional finance to any significant degree. To run extremely valuable contracts that affect tens or hundreds of millions of people takes space or military grade engineering reliability if chain rollbacks are completely out of the question. DeFi is only about five years old.