According to the graph, Apple is the main contributor to webkit. Do you have documentation to the contrary? I know it says Google was more involved in the last few years, but do you not think Apple will take up any slack with Google doing there own thing?
Look at the graph again. From April '08 on, Google has contributed over twice as much as Apple to Webkit.
While I seriously doubt Apple will just let Webkit slide without doing anything themselves, losing Google's support will probably sting a bit.