Coal is being switched out for natural gas here in the US... as a specific example, in 2012, see Plant McDonough right next to Atlanta, GA [1]. As I stated before, the reason is in part due to the effects on air quality (an externality), not just price. For Atlanta with this switch over, it has proven air quality improvements (and no one was surprised by this since it was a coal plant operating full blast right there next to Atlanta!).

I actually agree with you on several points. Agreed that the economics are uncertain, and agreed that scrubbing is very useful, and is definitely worth the cost vs. a full switch over to natural gas (but that doesn't ignore the fact that a switch to natural gas happens for other reasons). And I agree with you that natural gas has higher variance (historically at the least), and higher future expected prices, particularly with exports.

[1] http://www.georgiapower.com/about-energy/energy-sources/natu...