Comes down to whether they're willing to forfeit the potential 10-20k profit from investing that 15k wisely over 15 years, if it means they're burning less carbon
If your terminal goal was to see less carbon be burned, rather than to make a good financial decision, you could presumably accomplish more of that by taking the $15k (or ~$35k after investment) and using it to e.g. buy carbon offsets -- it's much cheaper and more effective to reduce emissions in the developing world than in countries like the US.