Eric Hellman has an interesting post on his blog "go to hellman" (great name, incidentally).
He postulates that by 2020 "the number of public libraries in 2020 would be half of what it is today. (And) the number of public library locations would increase by 50%." He goes on to describe a world with smaller, cheaper to run library outlets in different locations, and shuttering of some of the larger edifices. He thinks that the e-content revolution and the need to consolidate public services in times of restricted funding will help bring this about.
Hellman makes a pretty good case. We have already seen a lot of constriction in library budgets during the past few years, and given the state of the economy and the big black holes in government budgets, things aren't going to be rosy for a while.
So is the big box library a tool or a principle? If the building is a way to create a hub in your community, any space where people are willing to gather and share could work as a tool. If the building is a secular monument, a tourist attraction, or a way to keep a small town from sliding into oblivion, then the principle is a lot different.