You're right but I can't shake the feeling that if they ever become really popular the temptation to change that model will be huge. Maybe the DDG of today is principled enough not to do that (and it's also probably in their best interest at the moment since their most obvious feature compared to other big search engines is its privacy) but what about 10 years from now? Or 20 years? What happens if their growth starts stalling and the shareholders ask for more? Will they take the side of their freeloading users over the paying advertisers and investors? Principles tend to be soluble in a high-enough concentration of dollar bills.

After all there was a time where most of us trusted Google and their "Do No Evil" motto. And then eventually in morphed into "Do More Profit" and we have the corporation that we know today.

If I had the ability to know what a tech company was going to do 10 or 20 years in the future I wouldn't have sold off my Apple stock in the late 90s.

DDG may change how they do things, but I expect that if they do there will be someone else that shows up to make money by providing a value-added anonymizing wrapper on top of search performed by a larger company - and it may not even be on top of the duopoly as it exists right now (are there actual search providers in the US not wrapping Google or Bing?). This may particularly happen if a new company grows in India or China then its able to start spreading coverage to other parts of the world.