I agree with mostly this. Replace Maggie with Carl, and I think you've got your list of untouchables.
I honestly think a couple of seasons ago, that yes, "no one on the show is ever safe". I think AMC, the writers, creators, showrunner(s), et all know that this show is a goldmine. It's reached unprecedented viewership ratings (many times). The fan base is unreal. I think they know that there is no way they can kill an untouchable off at this point. Too much to lose.
I think the only way an untouchable will get killed off is if they want out of their contract.
Yeah, I wasn't sure if Carl should be added making it 7 untouchables, but I think you are right, I don't think they would kill of Carl.
I said it upthread, but I think the show has backed themselves into a corned. They have been known for killing off major characters in the first few seasons. It was shocking, but the fans were not really too attached to them yet. But now, at season 6, the fans are invested in these characters. Killing them off would be detrimental to the show. There are t-shirts that say "If Daryl dies we riot." But the fans have come to expect it. I think this was their answer to killing an untouchable without actually killing an untouchable. We all go through the emotions, anger, sadness etc. Then he comes back. Everyone is happy.