Proof of stake as dapp. Its value is in providing consensus to dapps.
The most popular type of dapp that requires consensus is cryptocurrency. Non-currency dapps also can use consensus, for example, https://en.wikipedia.org/wiki/CryptoKitties
So, proof of stake itself is a dapp, much like how the blockchain is the dapp store. Yeah, I can agree with that. The blockchain doesn't need PoS or PoW, it could exist if everyone was honest. Adding that layer of Proof of Stake is a dapp layered to secure and incentivise the network.
It is interesting to me because I'm kind of visioning this to be essentially a chain that can transfer unrestricted information at a cost of the coin which gets paid in a data tx transfer fee. Then a PoS dapp as a means of embedded incentive and security.
My only concern is, that ETH can hardly run CryptoKitties at its peak. That was only 1 popular dApp. The severe bottleneck that I really do believe that you should address.