You didn't kill it.
I have the same problem everytime after A power down.
You did something not correct with the commands.
Did you also do the 2 echo commands?
Yes, even though it didn't seem to be in DFU Mode, I did eventually try the 2 echo commands.
susb | grep 0483 still returns Bus 007 Device 003: ID 0483:5740 STMicroelectronics STM32F407
I don't know what else to try.