A counterbalance here though is that the global trend is making coal a poor investment. People building power plants do it for the return - and they won't do it if they can get a better return elsewhere.

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.