This could also be a perverse incentive reflected algorithmically (whether on purpose or not). Google has better data and makes more money when users are identified. So they have a vested interest in making users think that they will get worse results in Incognito mode.

Personalization is a double-edged sword. @pg once wrote about Google results becoming "what's true for you" rather than what's objectively true. And filter bubbles and subtle "personalization censorship" are also dangers. I think it's possible to have high quality results and privacy/anonymity, and it's not a binary choice. It's a challenge worth figuring out.