I'm not sure what you mean by "assume storage space will expand exponentially", since there is only a limited number of potential active crypto-currency users, making a small number of daily transactions (ignoring things like High Frequency Trading), recorded in a blockchain that grows linearly over time.
Would you say that the credit card network, or PayPal, has exponentially increasing storage requirements? It's possible for Bitcoin (for example) to be decentralised and useful to the world and only require linearly increasing storage space.
Fortunately it seems that storage technology will continue to scale linearly over the coming years too:
Would you say that the credit card network, or PayPal, has exponentially increasing storage requirements? It's possible for Bitcoin (for example) to be decentralised and useful to the world and only require linearly increasing storage space.
Fortunately it seems that storage technology will continue to scale linearly over the coming years too:
https://images.anandtech.com/doci/15064/seagate-roadmap.png