The latest sign that coal’s losing its sway in the U.S. power market: Vistra Energy Corp.—one of Texas’s largest power generators and coal plant owners—says the fossil fuel’s days are numbered.

“Coal is on its way out,” Vistra Chief Executive Officer Curtis Morgan said during a panel discussion at CERAWeek by IHS Markit on March 14. “More and more plants are being retired.”

And Morgan would know. The company has had to close four massive coal plants as their margins...