The money from Yahoo, now Google, is also a huge source of revenue for Mozilla (if not the main source). DuckDuckGo can't afford to fund the development of Firefox and Google isn't going to throw money at Firefox, if their search isn't the default... Well maybe a little, just to keep a competitor around and avoid anti-trust legislation.

Honestly I doubt the average user would notice if DuckDuckGo was the default, except perhaps for a few rephrasing of a few search queries.