That's a bit of a mistake. It doesn't rely on inefficiency it relies on economic costs. At the moment, hashing is a form of economic sunk cost. If you invest in hashpower, you can't split it without reducing your probability of success. It's an unforgable economic burden. Proof of stake architectures attempt to do the same thing with the currency of the network itself. There are lots of arguments about whether or not this is truly secure - but anyone on here telling you they know it's not is pretty much an idiot. There are tons of proof of stake coins operating successfully right now. There are no known attacks on these coins that allow you to do anything untoward (if there were, they'd be exploited on one of the many existing PoS coins).
No serious blockchain relies on proof-of-work. Bitcoin (being forked now) doesn't and ethereum (already forked twice) definitely doesn't.
Blockchains are a social construct and what ultimately protects them are the participants. Exactly like actual currencies they are subject to market forces; people either believe in and want the currency or they don't. It's this confidence that gives the currency strength.
More simply:
"On medium to long time scales, humans are quite good at consensus. Even if an adversary had access to unlimited hashing power, and came out with a 51% attack of any major blockchain that reverted even the last month of history, convincing the community that this chain is legitimate is much harder than just outrunning the main chain’s hashpower. They would need to subvert block explorers, every trusted member in the community, the New York Times, archive.org, and many other sources on the internet; all in all, convincing the world that the new attack chain is the one that came first in the information technology-dense 21st century is about as hard as convincing the world that the US moon landings never happened. These social considerations are what ultimately protect any blockchain in the long term, regardless of whether or not the blockchain’s community admits it (note that Bitcoin Core does admit this primacy of the social layer)." [1]
The subtlety here is making the distinction between confidence and resistance to attack. They're not the same thing at all. And what's interesting here is that, going off the model of real currencies, one could assume market forces will prevail here. Developed nations let their currencies float precisely because on the whole they're confident that the market will ultimately punish any malicious attacker who attempts to destroy the currency by selling large amounts of it.