As an electrical engineer working directly with the power grid, the choice to or not to retire coal baseload power plants is not simple or taken lightly. Many variables are involved: supply, demand, cost, forecasting, etc. As stated this is my perspective.
The power grid is no simple machine to be assumed to be constant. It is in fact the largest machine made by man; basically spanning and powering the entire country.
Renewable generation will hit its prime when energy storage becomes more affordable. That has not come yet and further research is required to being the cost down, it has to.
Combustible fuel is LIMITED and DIRTY, a shift is required. Not to mention how much we have and are altering the natural processes of our planet.
Back to coal generation: All of these coal generator closing notices, presumably old and nearing their end of useful life, have to be approved through the regional interconnect authorities. It is their responsibility to verify the actions do not negatively impact the stability of the grid; ensuring the voltage and frequency are stable.
They check that when required there is excess production elsewhere on the grid that can be tapped, leading to power exchanges to fulfill local needs. Everything has to be balanced: load = generation, now and into the future.
In summary know that these decisions are analyzed thoroughly.