What did Lewis mean when he predicted 10 to 12 years a tour across the continent would be viewed with as little concern as a voyage across the Atlantic?

About 150 years ago, the americans were hopeful and inspired by the phrase manifest destiny. According to history, Lewis and Clark discovered Oregon between 1804 and 1806, along with a small band of explorers. Lewis thought that in the course of 10 to 12 years many Americans will continue to follow the same road. However, this route Lewis and Clark had gone through would be too rugged for those much unexperienced travelers. In other words, his predictions were not really appealing to reality, more so, to what was pleasing to imagine. Nevertheless, in 1824 a passage that required low and flat wagons was discovered, by a young explorer called Jebediah Smith. This route gave way to many other settlers to find their fortune in Oregon.