Bitcoin has survived a rough 10 years, so many arguments against them, so this is not something new, in 2017 we have heard many anti-bitcoin arguments specially how it is negatively affecting the environment. So this argument is just a repeat, just by this time, Elon Musk is the face of it.
However, I don't think that bitcoin can be defeated or dethrone by a so called 'green' altcoin. They are still consuming electricity regardless of the source. So this kind of argument is flaw in the beginning. And not even a billionaire can stop Bitcoin now.
IMO The Green Altcoin that will dethrone Bitcoin is called Cardano.
It is 1.6 million times more energy efficient than Bitcoin.
It is a 3rd generation blockchain design, while bitcoin is a mere 1st generation blockchain design.
https://roadmap.cardano.org/en/Cardano is a groundbreaking proof-of-stake blockchain network, being developed into a decentralized application (DApp) development platform with a multi-asset ledger and verifiable smart contracts. Built with the rigor of high-assurance formal development methods, Cardano aims to achieve the scalability, interoperability, and sustainability needed for real-world applications. Cardano is designed to be the platform of choice for the large-scale, mission-critical DApps that will underpin the economy of the future.
Based on peer-reviewed academic research, Cardano has an ethos of openness and transparency. All of the research and technical specifications that underpin Cardano are publicly published, and all Cardano development activity is made available to the public. Cardano is designed by a global team of experts who are leaders in disciplines ranging from distributed systems to programming languages and game theory and is jointly developed by IOHK and partners. IOHK develops the technology, the Cardano Foundation is responsible for supervising development and promoting Cardano, while Emurgo drives commercial adoption. While we are building Cardano, we are merely custodians. When the network is fully decentralized it will belong to the community, and it will be the community who decide its future through advanced governance features.