It is easy to save energy when you are OK with a lower level of decentralization. I could probably run a payment processor on my computer alone that handles 100k transactions per second.

The proof of stake idea is like a ponzi scheme

There's a reason that ETH is barely hanging in there and will never catch up to Bitcoin. I doubt it will survive the next decade.

Barely hanging there? With the second largest market cap and 5th trade volume (for the coin itself, higher for the network)?