Comment by QuiEgo

Modern devices try to prevent this by cryptographically entangling the firmware on the flash to the chip - e.x. encrypting it with a device-unique key from a PUF. So if you replace the chip, it won't be able to decrypt the firmware on flash or boot.

The evil of the type of attack here is that the firmware with an exploit would be properly signed, so the firmware update systems on the chip would install it (and encrypt it with the PUF-based key) unless you have anti-rollback.

Of course, with a skilled enough attacker, anything is possible.