I honestly don't know enough to judge the financial sustainability of the company. To me it's more about incentives.

Right now it seems they're still growing really fast. What if tomorrow it's not the case anymore? What if their userbase reaches a plateau? Will they really still be morally strong enough to commit to user privacy or will they slowly start to give in more and more in order to increase their revenue? Does "do no evil" ring a bell?

On the other hand if they get money from the users themselves then the incentives are different, they might try to get more money by offering new services and they probably won't risk doing that that'll piss off their paying users.

Maybe I'm too cynical but in my mind even if ddg's intentions are pure now eventually they'll get bought or something and someone, some day will want to cash in. And for a free service cashing in means selling the userbase to the highest bidder.

But maybe I'm a bit naive, I guess that would still be possible even with paying users but I think the risk would be greater for them.

What data do you think DDG has on you now, that they could sell? Or is your concern that DDG could cell out on the promise of users, and that users would fail to notice a change in privacy-respecting behavior?

Precisely, yes. As with all internet services we can't really audit what's being collected as simple users, we have to trust them to "do no evil". If tomorrow ddg started logging my browsing history in order to better target ads for instance I probably wouldn't notice a thing.

This depends on the stack. If the cost grows faster than your revenue than it is a problem. If they have a lean stack where the cost grows slower than the revenue by the number of users they are good.