Now the coal industry is incredibly automated and mechanized, so I think the opposite is true today. Dump trucks with 400 tons capacity require just one operator, and even they are being automated in some ore mines with self-driving technology. If you look at aerial pictures of incredibly productive coal mines (in the Powder River Basin, for instance), there's just a few of these massive machines (which can be operated by one person) among many square miles. The coal industry only employs about 80,000 people in the US, in spite of the US accounting for a tenth of the world's coal production. That's less than a tenth the number employed in the 1920s when US coal production was about half of what it is today.

Mechanization and automation already did far more than eliminating the rest of the coal industry would.

Sounds like a milder type of conspiracy theory to me. Compared to coal, crude oil has higher energy density, provides better fuel to operate machines, and was less labor intensive to extract - isn't it quite enough for oil to expand into coal territory? Without some special political decision? Also, as we know democracy happened to places without mining industries at all.