I really don't understand this view. You are using proprietary software, you are always an N-line change away from someone doing something you don't like. This situation doesn't change this.
If you only use open source software and advocate for others to do the same, I would understand it more.
There is always a chain of trust that you end up depending on. OSS is not a panacea here.
(And besides, it's far more likely that this nefarious Government agency will just conceal a camera in your room to capture your fingers entering your passwords.)
And I don't understand why it has to be black and white, I think the N is very important in this formula and if it is low that is a cause for concern. Like an enemy building a missile silo on an island just off your coast but promising it's just for defense.
All arguments I see is along the lines of "Apple can technically do anything they want anyways so this doesn't matter". But maybe you're right and moving to FOSS is the only solution long-term, that's what I'm doing if Apple goes through with this.