"In these markets where the lowest prices from power producers are sold first, natural gas and renewables are underbidding coal and nuclear."

This is factually inaccurate. The problem with nuclear is that those reactors have to produce power all of the time. Powering down costs tens of millions of dollars every time it's done. Operators must avoid that, and everybody knows it, so nuclear power is sold at rock bottom prices, prices so low that nuclear energy comes close to being economically impracticable. Meanwhile, coal and oil power plants can be started and stopped for much, much lower cost, so they tend to come into their own during peak demand, when prices are at their highest. So, if we were to set up a system where all power is sold at the same price regardless of generation technology, we'd pull the rug out from under coal.