My government denominates my tax obligations in my government's currency. This includes taxes based on transactions of non-currency goods.
So there's always demand for my local currency so long as my government exists.
What demand is there for a decentralised digital currency? What motivates stable demand for it compared to the currency I can pay my taxes in and which my employer can be legally compelled by the courts to settle debts to me in?
With things like state receipts, light clients can rely on other network nodes to provide the state of transactions that were recently confirmed, without giving up security.
This does require that some nodes are processing and storing the most recent N blocks and their transactions. Today, non-mining full nodes do this for free out of kindness, but in the future, this could be offered as a paid service instead.
Proof of work is only one option for securing a chain. Research into proof of stake and delegated proof of stake will hopefully make PoW a thing of the past and drastically reduce the energy requirements to secure a blockchain.
The Ethereum team has two main focuses right now:
1. Proof of Stkae transition (Casper) 2. Implementing sharding
At the same time, teams are working on sidechain technology such as Plasma, Plasma Cash, and Loom which will essentially allow transactions to happen on another blockchain whose state is persisted in the root chain.
This not only benefits blockchain users (higher scalability, little to no fees), but is also good for the blockchain network since the data persisted to the main chain is only a set of merkle roots from the sidechain, no matter how many transactions the sidechain has.