To scale to visa/mastercard transactions per second, the blocks can never be big enough (if I remember correctly we need about 6Gig per block),
Never? Have you forgotten the mechanics of compound interest?
Internet bandwidth is growing by 24% CAGR. (
http://www.cisco.com/c/en/us/solutions/collateral/service-provider/visual-networking-index-vni/vni-hyperconnectivity-wp.html)
There is a ratio of 6000 between 1MB and 6GB. This is less than 12 doublings.
If 1MB was small enough for 2010, then 6GB will be small enough in the year 2045 or so. Dominant payment mechanism in 30 years? I like the sound of that. You really think adoption will move that fast?