The federal solar tax credit for residential systems was designed in such a way that it gradually decreases and completely disappears after 2021:The 30% tax credit applies as long as the home solar system is installed by December 31, 2019. Starting in 2020, the value of the tax credit will step down to 26%, and then again to 22% in 2021. After 2021, the tax credit for residential solar ends.

Can you think of a cost-based justification for gradually phasing out such a subsidy?