governments should have been falling over themselves to buy or otherwise subsidise expensive solar PV, because the more we bought, the faster the price would fall

That's quite a stretch, especially considering that there are plenty of examples where government subsidies and intervention distorts markets and makes them less efficient and more dysfunctional. I don't disagree with the general premise of the article: more could have been done to transition to renewable energy sooner. But it's really simplistic to say there was a clear, easy answer to this, and it simply involved more government spending on solar energy.