So you are suggesting them to maintain two different codebases during the transition rather than focus all their efforts to the new version to make the transition as short as possible?
IMHO they did the correct choice, every new version Opera will difference itself more and more of Chromium. You can continue to use Opera 12.16 until they take the decision to pull the trigger to completely kill 12.16 auto-updating it...
Remember the transition from Firefox 3.6 to the current Firefox, they didn't kill 3.6 until they reached version 12.