It's as though so many people saw Brexit and completely refused to learn anything from it. In the UK prior to Brexit the EU was often the scapegoat for things that politicians wanted but knew they couldn't get popular support for, the classic example is how the unpopular (and ultimately abortive) attempt to switch the UK fully to the metric system was very much seen as "petty-minded EU bureaucrats getting one over on the British public" in the tabloids when the reality was that almost all of the effort was Westminster's doing, mostly because business interests had been calling for it as a way to ease international trade.
The problem is that while falsely blaming an external outgroup for your problems isn't immediately dangerous, it can quickly spiral out of control. Around 2013 blaming the EU for bad policy-making on a wide range of things like immigration, the refugee crisis, a percieved decline in law and order, a creeping sense of political control over public discussion, and a whole list of other things reached its peak and the referendum was basically wrenched out of Cameron's hand. Eventually if you subvert a democracy by blaming your own unpopular choices on an outgroup, the inevitable question people start asking is "why don't we just get rid of $outgroup then? As you say, $outgroup cause all our problems so let's get rid of them and solve all our problems at once" which many billions of pounds and geopolitical bridges burned later is exactly what happened.
If nations as you put it "defer to external international conventions to be the bad-cop for wildly unpopular policy decisions" then things like Brexit are the inevitable result. Not only is it inherently wrong to sidestep democracy like that in my opinion, if supranational organisations are blamed for things that are unpopular then their influence will eventually be damaged by this. Organisations like the EU, the UN, and others aren't just limitless negativity sinks for domestic politics - all that political negativity has to go somewhere and eventually it just spills out.