The excess solar panels won't be built if there isn't demand for them. PoW mining creates that demand
1. We build 1TW of solar panels, PoW mining uses that 1TW to do useless work.
2. We don't build 1TW of solar panels, and don't do that additional 1TW of useless work.
Worlds 1 and 2 have the same impact on the electricity grid. But in world 1, we're emitting a whole lot of CO2 and consuming a whole lot of rare materials to build solar farms and electronics, while in world 2 we don't. Clearly world 2 is better for the environment?
If solar happens to not be cheap, very sustained, increased demand for non-solar sources increases the likelihood of non-solar success.