I think that you're neglecting legality/stability issues. Artisan mining can eke out a profit at low productivity rates if the laborers are operating in locatio

I think that you're neglecting legality/stability issues. Artisan mining can eke out a profit at low productivity rates if the laborers are operating in locations that are de jure protected from mining, but de facto have weak law enforcement. That's a big driver of artisan mining of gold, including that using child labor. High-efficiency, high-capital-investment mining processes also require a certain amount of expected stability to justify the up-front costs. If big multinationals think that an armed group is likely to take their equipment at gunpoint, or destroy it, they'll stay away and never be in direct competition with the capital-poor, low-productivity small scale methods.