Easy for the government to show significant negative outcome if it were legalized.
If this statement is true as it applies to hijackings then the comment that "it's not going to matter who's armed" in a hijacking is incorrect. It can't be both ways at once. If it doesn't matter who's armed then it can't also be true that allowing armed CHL holders aboard will likely result in a significantly negative outcome.
If you mean that there will likely be significant negative outcome in other scenarios not involving hijackings then that's another debate. I was careful to qualify my response so that it applied only to the hijacking scenario under discussion.