That might have been the case if Apple contributes nothing to Webkit, but they contribute a great deal as well. So Google benefits from Apple's work and Apple benefits from Google's work. There are many other companies that contribute to Webkit as well. Google was basically have to duplicate all of their work as their rendering engine diverges.
I can understand doing this if there was some really fundamentally broken thing about Webkit but I don't see it. Maybe it is hard to run every iframe in its own process but gimme a break, that's getting excessive there with the process separation. Why not run every element in its own process? Processes are free right? Even if this is some kind of killer feature of Blink, don't you think losing all that extra development effort is worth the tradeoff? I don't think so.