The population growth of the United States will never flatline for any appreciable amount of time. In other words, we'll never run out of workers. This is for two reasons:

(1) We border a rapidly developing nation to the south (Mexico), with significant economic ties at that border.

(2) The United States is, hands down, the most effective society at integrating immigrants and allowing them to be productive. We can always, always import more workers, and people will always, always want to come here.