Sure, you can't be 100% sure of anything. In the same vein, you can't even be sure that the world is real. The fact you can't have 100% assurances doesn't mean anything. What matters is how strong you make your assurances. A company having an existing system allowing for the installation of backdoored firmware is an example of something that shouldn't be possible. It should be possible to reasonably assure that someone cannot flash something onto a person's phone without either wiping the data or having the update signed by the user.