You could just pull the flash chip and image it. You would need to figure out how to get the key, but pulling the flash chip and reading it doesn't look too hard if you can use a heat gun. If you lived in Shenzhen you could go the market and buy a flash reader.
Strange Parts is youtube channel where the guy does this.
The filesystem is encrypted and the key is in the secure enclave. So pulling the chips and reading them directly no longer works.
I think this is how the FBI got into the bombers iPhone after losing the court case to try and force them into releasing an iOS version that allows unlimited pin tries.
The PIN or any other user secret can't decrypt the contents of the flash chip. So turning the device off and removing the flash just makes it infinitely more difficult.
I guess because now you have no way of exploiting any bugs that might've been exploitable. Within 7 days the phone will still try to talk to others connected to its port, after 7 days it'll just charge. I assume that "power off" will also disable data over the port until the phone gets unlocked?
Strange Parts is youtube channel where the guy does this.
https://www.youtube.com/watch?v=rHP-OPXK2ig