This is the million bitcoin question.
I would say that history has already proven that agreement on value is enough for something to hold value, it doesn't need to be backed by anything else than consensus. Without the waste of PoW and the capitalist risks of standard PoS, the most elegant and efficient solution to date seems to be Delegated Proof of Stake, the consensus protocol Steem runs on.
A month ago I would have agreed with you ,,, but I just learned of a 3 year old company (programmers) that is using an older tech called HashGraph which is mathematically even more secure than blockchain and the proof algorithm is even easier with less size.
http://www.swirlds.com
Well, Dan Larimer is pretty firm in his vision on Hashgraph and the limits of that technology. I don't know what technology will ultimately dominate the decentralized economy, whether that's DPOS, Tangle or Hashgraph... Could be something entirely new could be brought in by a big company like Google, Amazon, Facebook or Apple.
Be careful out there. Cryptoland is high risk land.