X でしかない === . . . has the only value (usefulness) of X
帯銃者は, もはや 脅威でしかありません
Gun-carriers are now nothing more than a threat.
Gun-carriers now constitute nothing more than a threat.
Gun-carriers now don't mean [ anything other than a threat ].
A gun-carrier now represents [ nothing but a threat ].
A gun-carrier now doesn't represent [ anything but a dangerous threat ].
I know that もはや+negative verb means "no longer",
Yes, that's right.
もはや doesn't have the meaning of NOT built into it.
もはや means that [something that was true] is not true (no longer true) after time T.
[something that was true] == carrying guns around (on one's person) had some value to society.
But this explanation doesn't match the following definitions completely.
Or maybe it does.
ある事態が変えられない ところ まで進んでいるさま。今となっては。もう。「―如何ともしがたい」「―これまで」
i wanted to add that ... (for the 2nd sense just above) it seems that もはや is used only when there is a negation (in the text) or there is a negative sense (to describe the current state).
もはや自由な時代になったんだよ is uncommon or impossible.
今や自由な時代になったんだよ is much better.