Moderator: igrr
It appears to me that there must be some sort of prepackaged firmware on ESP8266 modules that allows user firmware to be loaded via the Rx/Tx pins on the ESP8266.
f that firmware gets corrupted, then yes indeed the chip is dead. That's because the firmware is in ROM, not flash - and the only way it can get corrupted is for the chip to be physically damaged internally.
SPI has nothing to do with flashing firmware. It just happens that AVR chips share the same pins as SPI for their debug protocol.
You cannot replace the bootloader on an ESP8266. It is in ROM. It's unchangeable - except as mentioned earlier by physical trauma to the silicon itself.