> If electricity demand from PoW mining spurs new renewable plants to be built, is that bad for the environment?

Yes, obviously.

Even if every single miner pool built its own solar/wind plant to power 100% of its energy needs, that would still be horrible for the environment: building the power plant itself produces harm to the environment; and the space and work and money used to create the Bitcoin miner's power plant could have gone into replacing (closing down) a non-renewable power plant.

Silly arguments about pricing volatile electricity only work if we assume maximizing profit is the ultimate good or that PoW is the only way to use that excess power. In reality, if we want to avoid the worse catastrophes that our current economy is pushing the world towards, we have to stop looking at profit, and choose less profitable but more useful ways of handling volatility - batteries, long-distance transmission, etc.