Backbone operators in the US should not be allowed to connect to networks that connect to low trust countries.
Countries like Russia or China spend billions on controlling the flow of information on their own land. Countries like Iran go out of their way to blackhole the traffic whenever any disruptions or political violence happens in the country, and for every Nepal, where this backfired terribly, there's a dozen cases of countries doing that and getting away with it. And you're proposing we just help the authoritarians out by doing their dirty work for them.
Sure, let's do that! Give their propagandists a win, leave everyone who's in those countries now hang out to dry in an information black hole! Let the abuses perpetuated by their own governments go unseen and unheard! All to preserve the Good Web, For Good People Only.
Likely, the country that wanted to do this finds themselves isolated on their own network, not their target isolated from the internet. Even if that country as is large and powerful as the United States. Perhaps the answer might have been different, 20 years ago or even 15, but everything has changed and there's no going back.
And when other countries don't play ball? Then we shut down those backbones, and it's the United States that is isolate, not Russia (though please feel free to pick another target if you don't like Russia). No one's cutting off China, not without their economy dropping dead. Sure, maybe there's some country that you could do this to... but that country is so unimportant that they're probably already almost-cut-off anyway. You don't even get to to do this to a Brazil or Indonesia, let alone any country that matters.
Don't use that brain to cross the road