Easily? I hope these aren't the same economists who said there was no housing bubble and said there were no debt issues, etc...
There is nothing "easy" about predicting sales (how do you taken into account Android, overall economy, consumer knowledge, rightly or wrongly but newly formed bad will towards Apple, etc...). If it was easy, I suspect you would see businesses ran in an entirely different manner. I'm sure they can hire a economist to say any number they would like said, but the other side could do the same. Problem is that the other side won't have the money to compete. One of the big issues with hiring 'experts' and our legal system IMHO.