PoW can be as "carbon neutral" as centralized PoS if miners decide to use alternative energy sources (going green). No need to switch to PoS ever. But nothing's stopping you from forking off Bitcoin and creating a new PoS-based chain. That's the beauty of decentralization and open source, right? 
That's the beauty of the space in general.
Nobody stops you or regulates you to try something new out, even though it seems dumb / not working in the first place.
That's how many out there were created, in my opinion
