So what if Nephi and his brethren did not return to Jerusalem and obtain the brass plates? What if they had no scripture, or records of their ancestors?
Well, my guess is they would have become as the people in Zarahemla. The people of Zarahemla also came from Jarusalem, shortly after Lehi and his family left. They were also brought to the american continent, yet they had no scripture with them.
And at the time that Mosiah discovered them, they had become exceedingly numerous. Nevertheless, they had had many wars and serious contentions, and had fallen by the sword from time to time; and their language had become corrupted; and they had brought no records with them; and they denied the being of their Creator; and Mosiah, nor the people of Mosiah, could understand them. (Omni 1:17)
So within 300 years after leaving Jarusalem, their language became so corrupt that they could not be understood by the Nephites, and worst of all, they denied the existence of the Savior.
Wise purpose indeed that Nephi should obtain the brass plates.Google+