That's really the tipping point for solar - if it can provide a better return then coal plants. It's getting close, but the significant thing is that solar scales so much better then coal because you can build it anywhere - minimal environmental impact statements, no real supply lines other then grid access, minimal staffing and maintenance. If a straight up solar farm can provide a reliable return on investment over coal power, then no one is going to bother building coal plants any longer unless tax-breaks rain from the heavens (which they might).
And the kicker is very much that scalability aspect - solar doesn't even need to provide much of a better return then coal plants to beat them, it just needs to be deployable in such volume that the obvious investment strategy becomes "build solar".
And we're getting there - fast.
States NEED to take responsibility. If we are to change as a nation we need to stop looking at the globe and do a bit of introspection so that we can change internally. Otherwise if states are left out of the equation then the problem of climate change can never be solved.
While the federal government can possibly do that, and it is a huge possibility, trying to get the states to change at the grassroots is of far greater impact, and efficiency, as that is the Number 1 problem that USA faces. Not a lack of federal imposition, but a lack of grassroots understanding in America. And the one thing Americans hate perhaps the most is being told what to do by men in suits.
We also take into account that energy production is about large capital investments that aren't looking at tomorrow, but a decade or two ahead at the minimum. Building a new coal plant today isn't a bad idea because it'll be way to expensive to run in its first year, but because it'll be less and less attractive as time goes by, and will probably have its life cut short.
This is why on one hand, Trump's decisions aren't necessarily that hard hitting in the long run as long as the people building plants can expect his decisions today to be a historical aberration. It's the long run trendline that is important, and we can tell about that by what other stakeholders do.
That said, I'd not put much faith in most states caring about climate change individually. There's plenty of states that are deep red and where most energy today still comes from coal, and where candidates don't even bother mentioning an energy stance: Elections are about protecting religion and policemen and lower taxes. It's not that the states are relying on the federal government on the environment: If there's a mention of the environment, it's something like 'The EPA is too tough on our farmers'. This was a real talking point in Missouri's last election for governor, and guess what? the candidate that was in favor of letting farmers dump more chemicals into rivers won.
The federal government can force those states to make changes, but they'll have to be dragged kicking and screaming. It's good to have some states that lead the charge on this, but it's can't be the main way to do this. If this is decided state by state, the Midwest will be mostly powering itself with coal for decades.