If it makes sense installing solar panels when you have these 15K, then generally it makes sense to borrow the money as well (provided reasonable rates). The reason is that otherwise you would rather invest 15k into something (stock market, CD, whatever) than building the solar panel. Since the return rates from investment roughly follow borrowing rates, the situation is the same in both cases (borrowing vs already having money).
Of course, it does not mean that it is beneficial for everyone. People live in different states, with different laws, have different credit scores, so in each individual case it must be calculated. And in many cases it will work, and in others it will not.
But my main point is different. It is not like you are throwing away 15K. You are investing them, and will get return on investments. So, even if the returns are less than expected, you loose only the difference between 15K and the returns. So, if one really worries about climate change, they can act, and it does not cost 15K at the end to them. And in many cases it can be even financially beneficial.