It isn't even that complex an idea, you laid it out in 1 sentence. "If they have a backdoor, then their adversaries will eventually have it, too". They understand that just fine. If anything, you're the one who doesn't understand although I'd not push the point because the rules around political debate are weird and you might be humouring them.

The people in government have a hierarchy of threats. Social challenges from within their own nation always higher than those from foreign countries. Most politicians are perfectly happy to sacrifice national security to be able to see what their citizens are saying, they just won't articulate it in plain language because it sounds bad.

We can tell this because they're signing off on narratives around being "the good guys". Anyone pushing that sort of narrative spin isn't a good guy. That isn't how law enforcement works in practice and anyone in high political power knows it if they have even a shred of intellectual honesty; they've seen how the sausage is made and are well aware that the real world is all various shades of what happens when a family breaks down and siblings start fighting with each other.