“Theoretically, the five francs which the French iron producers receive (via the tariff) could be used more efficiently than the five francs saved by the French citizens using iron products.”

Not so. The five extra francs which the French iron producers receive go to cover the cost of being less efficient (e.g. the Belgian iron producers make 1 franc of profit on a 10 franc sale, and the French iron producers make one franc of profit on a 15 franc sale). In the end you have the exact same profit but customers have to pay more, thus a net loss.

Now, if you’re just saying that a particular company wants to increase their prices just to make more money, and they try to get legislation passed that blocks competition… well, there’s a lot going on there that’s hurts the economy the monopoly or oligarchy that emerges is going to be much less efficient than before, for one. And whenever you have dispersed costs and concentrated benefits you see what everyone tries to blame capitalism for: a few rich elite and the rest in poverty. That tends to lead to worse outcomes for everybody in the community, even the comparatively rich.