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.
Easily is relative term...
The current state is 15GB, but if this grows much larger the full nodes will probably come rather rare... Which might cause some other issues.
But these full nodes need to boostrap the new nodes at times and that uses bandwith in addition to the current network traffic. In worst cases some new nodes might be unable to ever catchup...