It is more nuanced than that. In a simple system this may be better than nothing, but consider what happens when the key that was used to sign that update has b

It is more nuanced than that. In a simple system this may be better than nothing, but consider what happens when the key that was used to sign that update has been compromised. Or, consider how an update with multiple payloads could be delivered, if each payload comes from a different vendor. Or, consider how a device can trust that an update has been signed by both someone authorized to make a release, and by someone who actually wrote the code in the release.