It's seldom necessary and often impractical to have full-blown shared-everything everywhere. There is a yawning gap going from simple key-value to the relative

It's seldom necessary and often impractical to have full-blown shared-everything everywhere. There is a yawning gap going from simple key-value to the relatively rich constraints and transaction processing SQL standardizes that is probably not necessary, and I think will be addressed with more programming (rich constraints and data models can exist over a smaller amount of data without trouble). Consider Cassanda column families vs. simple binary-string-to-binary-string systems, and it's already a significant advancement. I think that's the direction growth is going to take. Most of these large -- and particularly, global -- systems you speak of are eventually consistent also; they use the relational database as a potent backend member of a distributed system, able to check and deal with a great many constraint problems that can be localized.