But the deformed stamp could just become a different unique stamp.
You don't understand. Forensic labs can already match a firing pin to a fired case. Forensic labs can match a spent shell casing to a firearm through unique markings on the walls of the case as they are pressed into the chamber of the barrel. They can identify unique marks made by the extractor to a specific gun.
But all of the above requires that you already have the firearm and a spent casing in your possession.
Microstamping is designed to mark the shell casing with a unique serial number to identify the firearm that fired the casing without needing to have the gun in your possession.
The theory is that they can pick up a casing, identify the firearm, then perform a firearm trace back to the owner, and from there find the shooter.
In practice it doesn't work. The microstamping technology is not very reliable, and as I said it is too easy to alter the microstamping enough to make it unreadable.
If you bought a new firearm in the early 2000s, you would have found an envelope with two spent casings included in the box. There were a few states that required all new handguns sold to submit two sample spent casings to their police labs in order to build up a database of shell casing marks that could be used to identify the gun that shot the cases, but after 10 years and millions of dollars, not one crime was solved. Those states dropped the requirement.