"Let upstream determine the version that the distribution maintains."
So what does "maintain" mean when you complicate the packagescape with optional mini-repositories like Ubuntu specific PPAs? Aren't those PPA's "maintained?" They may not be official but does that matter to end users who are configuring their system to use PPAs? Doesn't the existence of PPAs break some of the fundamental assumptions here about the benefits of sticking with a specific upstream release. There are a lot of Ubuntu binaries built in launchpad PPAs, no one is talking about versioning-locking the PPA space are they? How many Ubuntu users configure at least one optional Ubuntu PPA for something in order get a newer version of some application? How widespread is the use of PPA binaries on Ubuntu LTS releases?
If Shuttleworth were really serious about the benefits of cross-distro version-locking, then I think he would need to rethink the policy on how PPAs are allowed to be used in the scope of just Ubuntu because they greatly undermine any sort of concept of an upstream preferred version which get priority attention.