We know from Snowden that metadata about who is communicating with whom, and when, is one of their most valuable data streams. While signal may not be able to turn over the contents of your messages, they absolutely retain a rich stream of metadata.
* they absolutely receive a rich stream of metadata.
This way, the users are only as secure as they want to be. The code would need to be signed using each one's public key, but we're talking about specialised software here.
Compare this to the de facto standard, where US corporations hold the private keys to everything hardware (off the top of my head, processor, UEFI) and everything software (SSL root keys, IP addresses, DNS).
We already have libpairip and play integrity on android, let's not bring it over to desktop processors.