Moore's law. Bitcoin's blockchain we can assume to be roughly double what it's current size is in a few more years time, and of course that will be determined by block size changes/capacity as well but we're essentially looking at a blockchain no matter how you look at it under 1 Terabyte even in 8 more years time. 1 Terabyte in eight more years should only cost roughly the price of a little cheap flash drive at that time.