> Coal today is just as clean as other forms of energy when you factor in all the externalities.
No, no it isn't. Coal is far dirtier -- even modern scrubbed plants (of which most aren't) emit mercury and other heavy metals, and SO2. Less than they used to, but more than gas, hydro, wind, nuclear, and solar, which emit none. Both gas and coal emit NOx (the others don't). Extracting coal from the ground is horribly messy -- just ask the good folks who like to drink water in West Virginia. Storing the coal waste is also horribly messy -- just ask the good folks who live along the Dan River in North Carolina and Virginia. Oh, and then there's those pesky CO2 emissions. Coal emits twice as much CO2 as gas per MWh, and of course hydro, nuclear, wind, and solar emit zero or virtually zero.
Coal is far, far dirtier than gas. Coal emits twice the CO2 as gas. In terms of environmental damage, the power plants aren't the same merely because they all have downsides.
> The environmentalists need to learn to quit when they achieve "good enough".
I suspect that they know to quit when they achieve good enough. After all, being an environmentalist is hard work -- the pay sucks if you're even lucky enough to get paid to do it. You're up against deep pockets all the time. The environmentalists won't quit until CO2 emissions are down 80%, and that won't happen so long as we're getting any electricity from coal.