If a Preimage Attack Were Possible, How Would It Affect the Target Difficulty Mechanism?

A successful preimage attack would render the target difficulty mechanism irrelevant. Miners would no longer need to perform massive computational work (brute-forcing nonces) to find a valid block hash.

Instead, they could instantly reverse the target hash to find a valid block header input. This would lead to blocks being found almost instantly, destroying the 10-minute block time, invalidating Proof-of-Work, and causing an immediate collapse of the Bitcoin security model.