I am a crypto-skeptic, and consciously chose not to work on it myself, but I still wouldn't want any investor, especially one as early stage as YC, to categorically rule out a sector based on the impact of its current technology, the state of the technology will be different when these companies have grown.
Eg with respect to the carbon impact of crypto specifically, the current chains are very unscalable for the exact same reason that they consume lots of carbon; while the per-user stats look bad now, any world in which these companies are used at the scale of web 2.0 platforms is necessarily a world where the carbon footprint per user is many orders of magnitude lower.