I think that the "fallback to Google" might actually tend to diminish consumer confidence in DDG. Every time you use it, you basically say to yourself "$newRiskyStrategy fails sometimes, we still need $oldReliableStrategy".

Instead, what might help DDG is a plugin that detects when you go past the first or second page of Google search results, and suggests that you might get better results on DDG. It's a little intrusive, but the mental nudge becomes "$oldReliableStrategy has flaws, try $newRiskyStrategy". You get a positive emotional interaction with DDG rather than "forcing" yourself to use it all of the time and "failing back" to Google.