You can prune the chain after you sync. As for the general issue of storing the state, there is research being done on "stateless clients". https://ethresear.c

You can prune the chain after you sync. As for the general issue of storing the state, there is research being done on "stateless clients". https://ethresear.ch/t/the-stateless-client-concept/172 https://ethresear.ch/t/the-stateless-client-concept/172 Instead of storing entire state, Validator just needs to the current state root, and transactions must prove the state that they access.