Actually, that was once true, but coal is no longer economical. Renewables are approaching the marginal operating cost for coal plants. It's already silly to build a new coal plant, because it costs more than other options. But once costs of renewables drop below the operating cost of coal, then you can save money by replacing existing plants. This is already true for less efficient coal plants and in the most sunny or windy places. Which is why you see coal plants being shut down.

The situation is complicated by lack of practical grid storage, but using gas plants that can spin up and down quickly plus renewables is currently more economical than new coal. Partly because fracked gas is cheap.