That app already exists. It's called TM SGNL. The Department of War used it. It sent all their messages in plain-text to an Israeli server that was leaking memory dumps to the public internet (a la Heartbleed), 600GB of which were collected by hackers and sold on the dark web. Worst case scenario. That's not a fantasy, that literally happened. Do you still trust Signal?

That just reinforces what I said. It was not a problem in Signal, it was a problem with this third-party.