take…for granted 是英语中的惯用搭配，意思是“认为……是当然的”“想当然地认为……”。如：

I took it for granted that you would stay with us. 我肯定你会住在我们这里的。

I took it for granted that we’d be invited. 我想当然地以为我们会被邀请。

Losing my job taught me never to take anything for granted. 失业使我认识到决不能认为什么事都是理所当然的。

We must not take it for granted that the board of the directors will approve of the investment plan. 我们决不可想当然地认为董事会一定会批准这个投资计划的。

有时也表示“(因视作当然而)对……不予重视”。如：

He is so busy with his job that he takes his family for granted. 他工作太忙，以致不把家当回事儿。

你的句子意思：

Americans stopped taking prosperity for granted.  美国不再认为繁荣是理所当然的事。