Drupal project lead Dries Buytaert recently launched a community discussion by asking "should we decouple Drupal with a client-side framework?".
Regarding progressive and full decoupling: there are two important competing forces to balance. The first is that I'm a big believer in steady evolutionary improvement over radical rewrites, which is an argument in favor of progressive decoupling. Drupal's community and technology are both built on PHP-driven rendering, so it's very clear that PHP-driven rendering needs to be first-class.
Tactically, I think the best way to discover what's going to work for the Drupal community is to build one or more prototypes of Drupal's own admin interface as decoupled apps. The admin interface is very clearly an application (as opposed to a content page), and its complexity makes it a great test case.