I so agree. Delicious is one of the most interesting datasets on the internet for precisely the reason you cited: it includes nearly every web page of value that exists, all tagged and linked in endlessly rich ways. I still use Delicious nearly every day for this reason -- despite how slow it is and how badly Yahoo messed it up -- yet I've only ever scratched its surface. There's so much that could be done with it in the right hands. It pains me to think of it going away. Actually, it's tragic.
Why don't they spin it off into a startup? Just open-sourcing the code would be useless and even the data wouldn't hold its value for very long without the application and community continuing. But a startup would have a fighting chance to do something great with it.