The blockchain is already 15 GiB. Bitcoin can't replace credit cards if we don't solve this problem.
Why is this a problem? Consumer flash memory devices that can hold 16G can be found for less than $10. This eventually gets cheaper. Mass produced devices can hold a ROM of the current history of the blockchain for even cheaper.
That's $0.625 per GB, that's insane. I buy at ~ $0.05 per GB, and, that's insanely expensive for just long-term storage of data that can easily be reacquired, most low-end hard-drives you can buy for ~ $0.03 per GB.