Having thought about this a bit further, I can see idea behind Say's Law. The neoclassical view is that the only way to grow an economy operating at full potential output in the long run is to increase its productive capacity. This can be through additional or better plant and equipment, it can be through (hopefully) a more educated or more skilled work force, it can be through better infrastructure or it can be through successful new techniques (for instance, the automation of the assembly line, or before that, the non automated assembly line, and before that the use of standardized parts.)
Beyond any question, productivity is essential to solving some basic problems. But with that productivity one must have a market to decide what to produce -- and just as importantly, what
not to produce. Commies were able to rush through the early stages of industrial development, only to get stuck there producing stuff nobody wanted. The central planners of the USSR and other related entities made great mistakes in connecting production with human desires.
But as we enter an era of surplus in almost everything, we might no longer solve many problems with greater productivity.
What happens is that those who make money (workers and suppliers) themselves get funds for buying stuff. But just because I work in a pharmaceutical facility that produces insulin does not mean that I will buy it if I am not a diabetic.
Under the circumstances, all that could save 'coal' as an industry is subsidies from the government or mandated use contrary to market costs.
