You're missing the point of a carbon tax. The tax is meant to speed the end of fossil fuel use. And really it's natural gas that killed coal, so you're going after the wrong target.
The current market forces point to a direction of renewables, natural gas, and whatever nuclear remains operational (with no new nuclear plants). That's not a bad plan for the US for right now. However, natural gas in the US is currently 1/3 to 1/4 the cost of any other natural gas in the world. It is exceptionally, and historically cheap. Various people estimate that this low-pricing situation will last between 15 and 100 years. My personal opinion is that it is difficult to make estimates on that kind of timeframe.
Regardless, if natural gas in the US ever approaches the cost of natural gas elsewhere in the world, US consumers would be in for a very rude awakening on their utility bills. My personal opinion is that we should not eliminate these plants entirely. It isn't wrong to let market forces dictate our choices, but we should hedge against unfavorable market changes in the future.
Disclaimer- I'm "in the industry", my customers are roughly 60% gas and 40% coal.