The upside is - damange to the environment aside - oil is a universal currency. Lowing the cost of energy is like printing money and mailing everyone a cheque; without the inflation. (It's also why oil is has been used as an economic weapon). Lowing the cost of energy impacts everyone. It's like being The Fed and lowing interest rates, but better since Joe & Jane Q Public benefit every time they pull up to the pump.…
Fracking is also better for carbon emissions and helped price out coal, which is a huge environmental benefit. Nothing is black and white
Coal was a fraction of market share. It was on its way out regardless. However, a lower oil price increases consumption across the board. Thus it wouldn't take much increase for oil's aggregate pollution to exceed oil + coal.
Yes maybe on a per kilowatt basis oil is better than coal. But given the usage of oil, that is the scale, the price drop triggered increases in consumption could in fact cover and then exceed the gain from less coal.