Yeah but you can have open borders (+ foreign stability) together with mercantilism in order to mitigate the bad effects of a low imports/exports score.
True, although in that case you'd have to question how much money you'd make from those foreign trade routes (combined with Customs Houses) compared to with mercantilism. And then you could compare it to what you get from a free specialist.
Still, late game I favor communism or environmentalism purely to dodge great depressions suddenly collapsing a sprawling empire. I tend to Free Market with smaller to medium empires for the trade routes.
Lets say a medium to large city has 2 foreign trade routes at 8 gold. Some will be bigger and some will be smaller, so I'm just spitballing numbers here.
Switching to mercantilism gives you a free specialist which could be say 3 hammers. Obviously you could pick a lot of different specialists, and I don't want to spend time on each individual one, so I'm just thinking about engineer for now to simplify.
You're paying 8 gold to 3 hammers.
Of course, maybe only half of your cities have a foreign trade route, so on average, its 4 gold to 3 hammers.
So, then what's the difference of that on GDP? Probably small, but it depends on the weighting of GDP differences vs. imports/exports.
I think overall this just strengthens my reasoning for staying in Mercantilism until I adopt Communism or Environmentalism for a large empire. I always thought the differences would be larger, because I would have those trade routes, but it seems that overall those trade routes hurt more than they help, stability wise.