Bill Gates would have to believe his efforts were worthwhile in order to keep himself motivated to try. He's essentially "all-in" on the Gates Foundation. His pronouncement on eliminating poverty is an aspiration he phrases as a "fact" and we are taken aback by the audacious claim, primarily because he's been wildly successful before, so hey, lightning might strike twice.

What if? And why discourage him?

It doesn't seem likely to me, but I don't have billions to spend proving my theories. When I think in terms of probably outcomes, it seems more likely that the US will regress towards the global mean, rather than remaining as a permanent statistical outlier.

I also keep in mind that "poverty" has a way of becoming a dynamic definition. In the US, it's becoming that anyone below the 33rd percentile is poor by that definition. So poverty is always with us in that frame of mind.