For instance, you could remove mining to save energy and create a closed loop, recursive, PoS system, but without the external entropy of mining, you now have a permissioned ledger and not a real decentralized currency.  It has no real fault or state recovery since it needs to reference the parts that have already failed in order to continue.  With Bitcoin PoW, there really is no terminal failure unless the cryptography itself becomes compromised.  That's why Bitcoin is more valuable than all the alternative methods you see.  A global economy can't go from working to dead at the drop of a hat with no way to fix it.