Good points. My thought is that in an industrial age, magic users would come to the same conclusion as everyone else—that wealth is now firmly the truest source of political and social power. In my world, fire wizard labor wouldn't be cheaper than coal—especially coming from an age where they held nobility status. Rather, they would become the most elite scientists and engineers, helping to magically enhance its efficacy. In fact, they may well be the ones to push for it in the first place, as any fire wizard offering magically-enhanced coal in place of traditional in-person wizards or mundane coal would blast their competitors away because of economies of scale. I think it would generally mirror what happened in our world, just with the fact of x + magic = x but better tacked on to everything.