Sorry this has taken so long to get posted.
One of the reasons this announcement has generated such a strong response from Carbon developers is that most of us have already spent way too much time rewriting our applications to accommodate changes in the Mac OS. The biggie for many was carbonizing an existing application from Mac OS 9 so it would run on Mac OS X. But there as also the move to the HIView model and compositing windows, converting windows to nibs (an unnecessarily painful process), adopting the new File Manager APIs, adding support for Unicode, moving our work from CodeWarrior to Xcode, and updating our code to work on Intel-based Macs. Note that almost none of this resulted in a difference a user could see, so this all had to be in addition to new features and enhancements. It was brutal for many people, depending on the complexity of your application had how well you'd kept your code up-to-date through Mac OS 9.
Now Apple wants us to transition to Cocoa, and once again they haven't done much to make the path an easy one. For example, Interface Builder can't convert a Carbon nib to a Cocoa nib, and a post on the carbon-dev list by an Apple engineer a few days ago suggests that no one at Apple had even thought of providing that. So all those fools who believed Apple and fought with IB to create Carbon nibs are going to have to recreate them all over again from scratch now if they want to switch to Cocoa.