> Successful at what? Experience says that they won't be more successful at being good to other people.
No, see, you're just challenging the axiom.
The reason for the thought experiment is that nobody starts off by knowing things like that one way or the other. You don't know that it is true but you also can't assume that it isn't just because it being true would be inconvenient. You have to consider the possibility that not every fact is advantageous to your in-group, and be willing to accept the cases where that actually happens. Because it will.
Just because somebody says something you don't like doesn't inherently mean that they're wrong. We shouldn't punish people for saying things that could be the truth. If you think they're wrong then you make your case. Everybody else gets to hear you both and make up their own mind. Censoring the opposition is cheating.
> Your line of reasoning also leads one to say that black people shouldn't reproduce because their children are statistically less likely to become wealthy. That doesn't strike me as being an honest ethical framework.
No, it doesn't, because existence isn't the same thing as adoption. The average black kid may have a harder life than the average white kid but that doesn't mean given the choice they would choose not to be born.
Adoption isn't a matter of the kid existing or not; the kid exists. The question is who gets the kid. And the answer should be based on what's best for the kid. You can have a debate about how to define "best" but for any given definition, that's an empirical question with a factual answer. Which means we actually need the facts. And if the facts turn out to be convenient, that's great. But if they're not, they're not. The other side gets to make their case.
You have to be willing to lose when you're wrong because it's the only way you can win when you're right. Anything else is just a war whose victor is decided by might rather than truth.