Because blockchain is decentralized, what would happen if there was no cryptography anymore to protect the blockchain? I know it would not be used anymore but why? "Hackers" could reverse transactions but I can't think of anything else...
Blockchain is a decentralized append-only ledger.
If it does not use cryptography to achieve the goal of being append-only, then it would likely not provide any real guarantee of being append-only. It would only provide such a guarantee as long as it was used as prescribed.
For instance, we could create an append-only data structure in a program, and simply program the API to refuse to make any modifications other than appending new data.
However, that does not guarantee that no other modifications could be made to the underlying data, there could exist many ways to work around the provided API to insert/remove/etc. as desired - if you don't use the provided API you can do anything you want to the underlying data. If there is no form of cryptography protecting the integrity of the information, then any peers that attempt to read this data structure have no way of knowing it has been tampered with.
Without cryptography, "append-only" is more of a suggestion/recommendation rather than a matter of technical capability. It is merely inconvenient rather than computationally intractable.