> If Watsi didn't exist, many of the patients on our site would have likely died. How do you quantify that impact?
Roughly, by comparing it to the number of people who die in other hypothetical scenarios. You can't get exact numbers, but you can do a damn sight better than saying "it can't be done".
> How do you prove that all the donations made via Watsi would have been allocated to some other cause had we not existed?
Of course they wouldn't. But what if, instead of creating Watsi, you had put the same effort into advocating for more efficient charities? (This is a serious question. I actually wouldn't be completely shocked if it turns out that that would have been less effective.)
> Nyaya Health, which was rated one of GiveWell's "Standout Organizations" last year.
However, they didn't estimate its cost-effectiveness. And on that metric... I go to your website and see that $1000 can stop someone from maybe becoming infertile, and that just doesn't seem like it can possibly win against malaria nets. (Looking deeper, that seems like one of the lowest value treatments available, but I don't expect the others to win either.)
Don't get me wrong, I'm glad you guys are doing this. I'm not convinced you're doing the best you possibly could, but you're doing far better than me. I absolutely think your impact on the world is massively positive. But I don't anticipate donating to you myself any time soon.