The current price of ETH is completely unsustainable.
There is not a single dApp that has a huge userbase and I´m also
not seeing the emergence of one in the foreseeable future. [...]
Well... there is
CryptoKitties 
The switch to PoS keeps getting postponed, which could be an indication
that there are unsolved problems. Besides, it is questionable whether Proof-of-Stake
is even technically viable at all. There is a reason why Satoshi Nakamoto decided to use
Proof-of-Work for Bitcoin.
Yeah, I don't think they will ever switch to some PoS, but I may be wrong... There are though some "new" (2016) academic developments with respect to PoS: it's not trivial but it is feasible. This doesn't mean that PoS will replace PoW because they have a different set of strong/weak points, so I think they will just coexist. (I'm also curious about this new
proof of space and time, which I believe could be a real alternative to PoW)