That would be a different experiment, as it would also be testing whether people would, for example:
Predict from sight/marketing that BFWHAS was tastier than ice cream
Believe that buying BFWHAS would be better for their status
Those factors could go either way, but they’d disrupt a pure test of this part of the alien’s predictions:
Future humans will enjoy, say, raw bear fat covered with honey, sprinkled with salt flakes.
I still expect ice cream would win a blind taste test, but I didn’t predict these survey results.
In “less time than average”, which average? In the “create a child that they know will die of cancer at 10″ thought experiment, the child is destined to die sooner than other children born that day. Whereas in the “human extinction in 10 years” thought experiment, the child is destined to die at about the same time as other children born that day, so they are not going to have “less time than average” in that sense. Those thought experiments have different answers by my intuitions.
My intuitions about what children think are also different to yours. There are many children who are angry at adults for the state of the world into which they were born. Mostly they are not angry at their parents for creating them in a fallen world. Children have many different takes on the Adam and Eve story, but I’ve not heard a child argue that Adam and Eve should not have had children because their children’s lives would necessarily be shorter and less pleasant than their own had been.