But trade was a tiny percentage of either country's economy: these countries had total annual exports something close to 5% of their GDPs. In contrast, for example the USA now exports about 20% of its GDP; China, about 35%.

Also, industrial goods today have complex global supply chains, unlike in the past. In 1914 perhaps the worst French import that Germans lost was something like luxury foodstuffs. Today it might be more like, nobody can build X smartphone or airplane because one of its critical parts is only made in one place, and building an identical plant on this side of the war border will take several years. There's a multiplier on the economic damages here, because losing the ability to trade for a $5 chip might mean your economy loses a $500 smartphone.

That's not to say this makes a big war impossible, but shutting down trade would be vastly more painful today than back then.