Got into a debate/argument in class

This is not only related to BTC. I'm from Japan, Waseda University. I had a class this morning where I presented blockchain technology and was bombarded by questions/doubts by three American exchange students. They claimed that a bitcoin transaction consumes the same amount of energy that some countries in Scandinavia does, and also how all "use-cases" of cryptocurrency are just speculation, saying it's basically the same as a Ponzi scheme. Of course, I fought back by saying the shit that they were saying doesn't even make sense. If it were true, then the world would've died long ago after millions of transactions every day. But how should I better answer these questions the next time I face the same situation?