No, at that point in time currency will be human body parts...

Nothing else left to eat?

I imagine the best currency to hold would be the yuan

Good luck using your Visa, Mastercard, Apple Pay, Google Pay, {insert corporation} Pay also.

I wouldn't trade it for gold, paper or sparkly stones either.. Iron tools on the other hand..

The irony is that crypto, due to its tremendous energy-consumption, does contribute to the collapse of mankind, so crypto-miners and -traders actually contribute to the opposite of security.

Energy consumption by itself does not "contribute to the collapse of mankind". Be specific about what you mean.

@Erik 🮱 The causality is straightforward and well known. Energy consumption leads to carbonisation of the atmosphere which leads to climate change, which leads to so called natural disasters, including drought, floods and cyclones. As the environmental pressures increase, the likelihood of societal collapse also increases.