I believe the pants that discussed here are owned by the company I work for. I'm on the Nuclear end, but we hear all about the coal end.
Simply the demand for electric is down. And the cost to run these plants is up. It makes sense to close them. We've been upgrading some of the better plants so they can make more power more efficienty and reduce emmisions. The plants that are closing are not worth upgrading.
Natural Gas is cheap right now (which means the profit from electric is down, they are connected, it's a strange connection but our profits follow the price of NG) There is discussion of converting to NG, but that is also expensive and takes several years. The better solution is to just invest in the better performing plants and make them more efficient.
Electric demand has been down since the crash a few years ago, it has not recovered completely, and we don't expect it to. Companies either are not producing as much, or have also improved their efficiency and are not using as much. Especially in the auto industry, which is one of the biggest customers of our company.
And "base load" is maintained by the nuclear plants (and small percentage by hydro). Coal plants are secondary. NG units are peaking units. Wind and other sources are used when ever they are available (replaces the coal MW)
Nuclear plants are always at 100% power (unless down for maintenance) We always fluctuate the coal plants according to the load demand. As an example, we often reduce MW supplied in the evenings when demand from industry is lower.