It's not a strawman. It's what would have to happen according to the GPLv3. The fact that you could design a system that allows for user-replaceable keys doesn'

It's not a strawman. It's what would have to happen according to the GPLv3. The fact that you could design a system that allows for user-replaceable keys doesn't change the fact that iOS is not that system, and that if GPLv3 code gets into iOS then, according to the terms of that license, owners of existing devices would need to be allowed to replace the code. And the only way to do that is to release the root signing keys. You can't say "well they could design a future device to allow this", because that hypothetical future device isn't what we have today.