This is an understandable reaction, but the security margin on new crypto is way higher than old crypto. Roughly speaking, we went from "I guess if a state-level actor dedicated all their resources to this for a few decades, they could probably brute-force it" to "Even if you broke 9 out of 10 rounds in this algorithm, you'd still need to harness the energy of every star in the universe for 10 billion years to brute-force it."

Most algorithms today have been "attacked" in the sense that there are tricks we can do that allow us to recover the key faster than a simple brute-force attack. But "faster" usually means doing something like 2^100 operations instead of 2^128 -- still far beyond the realm of practicality.

It's telling that cryptographers are now seriously discussing reducing the security of various algorithms: https://eprint.iacr.org/2019/1492

No, "they" don't. SHA1 collisions had been "in the wind" for a while, they had been in sight ever since MD5 started showing signs of clear weakness in the early '00s. Wikipedia has a Rivest quote about it from 2005. There is nothing like that for SHA2, although attacks are improving.

> What are the chances of a collision if we simultaneously use multiple hashes

Define "simultaneous". Shipping twice the hashes for each piece seems a big waste of space. If you mean re-hashing hashes, it's just a waste of cpu power, since an attacker only has to break one or the other to get in a position to poison data.